houses arent the only place where insulation can be seen in our world. two kinds of animals — birds and mammals — maintain a constant body temperature despite the temperature of their surroundings. both have evolved methods to control the flow of heat into and out of their bodies. part of these strategies involve the use of insulating materials — fat, feathers, and fur — that serve to slow down the heat flow. because most of the time an animals body is warmer than the environment, the most common situation is one in which the insulation works to keep heat in.
whales, walruses, and seals are examples of animals that have thick layers of fat to insulate them from the cold arctic waters in which they swim. fat is a poor conductor of heat and plays much the same role in their bodies as the fiberglass insulation in your attic.
feathers are another kind of insulation. they are made of light, hollow tubes connected to each other by an array of small interlocking spikes. they have some insulating properties themselves, but their main effect comes from the fact that they trap air next to the body. this stationary air is a rather good insulator. for instance, in winter, a house sparrow has about 3,500 feathers, which maintain the birds normal temperature even in below - freezing weather. birds often react to extreme cold by contracting muscles in their skin so that the feathers fluff out. this increases the thickness — and hence the insulating power — of the layer of trapped air. incidentally, birds need insulation more than we do because their normal body temperature is 106°f.
hair (or fur) is actually made up of dead cells similar to those in the outer layer of the skin. like feathers, hair serves as an insulator in its own right and traps a layer of air near the body. in some animals (for example, polar bears) the insulating power of the hair is increased because each hair contains tiny bubbles of trapped air. the reflection of light from these bubbles makes polar bear fur appear white — the strands of hair are actually semitransparent.
hair grows from follicles in the skin, and small muscles allow animals to make their hair stand up to increase its insulating power. human beings, who evolved in a warm climate, have lost much of their body hair as well as the ability to make most of it stand up. there is a reminder of our mammalian nature, however, in the phenomenon of “goose bumps,” which is the attempt by muscles in the skin to make the nonexistent hair stand up.
which sentence best states the main idea of paragraph 3?
a according to many scientists, feathers were not originally developed for flight.
b the light hollow tubes in feathers have insulating properties themselves.
c feathers keep in heat with their insulating properties and by trapping air.
d in reality, feathers most important function is for flight.

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Accepted Answer

# Brief Explanations:
To determine the main idea of paragraph 3, we analyze each option:
- Option A: The paragraph does not discuss the origin of feathers (whether for flight or not). It focuses on their insulating properties. Eliminate A.
- Option B: The paragraph states feathers have insulating properties and trap air (via the small interlocking spikes and hollow tubes trapping air next to the body). This matches the content.
- Option C: The paragraph says feathers trap air (not just "keep in heat with their insulating properties and by trapping air" – the main point is about their structure trapping air for insulation, but the option's wording is less accurate than B? Wait, no, let's recheck. Wait, the paragraph says feathers are made of light hollow tubes connected by interlocking spikes, trapping air next to the body (good insulator). Then it talks about birds fluffing feathers (muscles) to increase thickness (trapped air) for more insulation. So the main idea is about feathers' insulating properties and trapping air? Wait, no, option B says "The light hollow tubes in feathers have insulating properties themselves." But the text says the main effect is from trapping air, not the tubes themselves. Wait, maybe I misread. Let's re-express the paragraph: Feathers are another insulation. Made of light hollow tubes connected by interlocking spikes. They have insulating properties, but main effect is trapping air next to body (good insulator). Then birds fluff feathers (muscles) to increase thickness (trapped air) for more insulation. So the main idea is feathers' insulating properties (including trapping air). Wait, option C: "feathers keep in heat with their insulating properties and by trapping air." Option B: "The light hollow tubes in feathers have insulating properties themselves." The text says the tubes are part of the structure that traps air, but the main insulation is from the trapped air. However, the question is which sentence best states the main idea. Let's check the options again. Wait, the paragraph starts by saying feathers are another insulation, made of light hollow tubes with interlocking spikes, which trap air (good insulator). Then explains how birds use muscles to fluff feathers (increase trapped air) for more insulation. So the main idea is about feathers' insulating properties (trapping air). Now, option C says "feathers keep in heat with their insulating properties and by trapping air." Option B says the hollow tubes have insulating properties. But the text says the main effect is trapping air, not the tubes themselves. Wait, maybe the key is that the paragraph's main idea is that feathers have insulating properties (from trapping air) and their structure (hollow tubes, spikes) helps trap air. But among the options, option C is more accurate? Wait, no, let's check the options:

Option A: About feathers' origin (flight) – paragraph doesn't discuss that. Eliminate.

Option B: Says hollow tubes have insulating properties. But the text says the main insulation is from trapped air, not the tubes. So B is incorrect.

Option C: "feathers keep in heat with their insulating properties and by trapping air." The paragraph says feathers trap air (good insulator) to keep heat in. This matches.

Option D: Says feathers' main function is flight – paragraph is about insulation, not flight. Eliminate D.

Wait, I think I made a mistake earlier. The correct answer should be C? Wait, no, let's re-read the paragraph:

"Feathers are another kind of insulation. They are made of light, hollow tubes connected to each other by an array of small interlocking spikes. They have some insulating properties themselves, but their main effect comes from the fact that they trap air next to the body. This stationary air is a rather good insulator. For instance, in winter, a house sparrow has about 3,500 feathers, which maintain the bird’s normal temperature even in below - freezing weather. Birds often react to extreme cold by contracting muscles in their skin so that the feathers fluff out. This increases the thickness — and hence the insulating power — of the layer of trapped air. Incidentally, birds need insulation more than we do because their normal body temperature is 106°F."

So the main idea is that feathers are an insulator, they trap air (their main effect), and have some insulating properties themselves (the tubes). But the key points are: feathers insulate by trapping air (via their structure) and have insulating properties. Now, the options:

Option B: "The light hollow tubes in feathers have insulating properties themselves." – this is part of the paragraph (they have some insulating properties themselves), but the main idea is also about trapping air. However, the question is which sentence best states the main idea. Let's check the options again:

- Option B: Focuses on the hollow tubes' insulating properties.

- Option C: Focuses on feathers keeping heat via insulating properties and trapping air.

The paragraph says "they have some insulating properties themselves, but their main effect comes from the fact that they trap air next to the body." So the main idea includes both: their own insulating properties and trapping air. But option C says "feathers keep in heat with their insulating properties and by trapping air." Option B says the hollow tubes have insulating properties. Which is better?

Wait, the paragraph's first sentence: "Feathers are another kind of insulation." Then it explains their structure (hollow tubes, interlocking spikes) that traps air (good insulator). Then how birds use muscles to fluff feathers (increase trapped air) for more insulation. So the main idea is that feathers are an insulator, using their structure (hollow tubes, spikes) to trap air (which is a good insulator) to keep heat in. So option C: "feathers keep in heat with their insulating properties and by trapping air" – this captures the main idea (insulating properties + trapping air). Option B: "The light hollow tubes in feathers have insulating properties themselves" – this is a detail, not the main idea. The main idea is about feathers as a whole trapping air for insulation, not just the tubes. So the correct answer is C? Wait, but earlier I thought B, but now I'm confused. Wait, let's check the options again:

Option A: Incorrect (origin of feathers not discussed).

Option B: Talks about hollow tubes' properties – but the main insulation is from trapped air, not the tubes themselves. So B is incorrect.

Option C: Says feathers keep in heat with their insulating properties and by trapping air. The paragraph says feathers trap air (good insulator) to keep heat in, and they have insulating properties. So this is correct.

Option D: Incorrect (main function not flight).

So the correct answer is C? Wait, no, the original text's paragraph 3 (assuming the third paragraph is about feathers) – wait, the user's image: the paragraph starts with "Feathers are another kind of insulation..." So the main idea is feathers' role in insulation (trapping air, etc.). So option C: "feathers keep in heat with their insulating properties and by trapping air" – this is the best. Wait, but maybe I made a mistake. Alternatively, maybe the correct answer is B. Wait, let's check the options again.

Wait, the paragraph says: "Feathers are another kind of insulation. They are made of light, hollow tubes connected to each other by an array of small interlocking spikes. They have some insulating properties themselves, but their main effect comes from the fact that they trap air next to the body. This stationary air is a rather good insulator."

So the two key points: 1. Feathers have insulating properties. 2. They trap air (which is a good insulator) next to the body. So option C says "feathers keep in heat with their insulating properties and by trapping air" – this includes both. Option B says "The light hollow tubes in feathers have insulating properties themselves" – this is only the first part (their own properties), not the main effect (trapping air). So option C is better. Wait, but maybe the answer is B? No, because the main effect is trapping air, not the tubes' properties. So the correct answer is C? Wait, no, let's check the options again.

Wait, the question is "Which sentence best states the main idea of paragraph 3?"

Paragraph 3 (about feathers):

- Feathers are insulation.

- Structure: light hollow tubes + interlocking spikes.

- Insulating properties: some from tubes, main from trapping air next to body (good insulator).

- Birds fluff feathers (muscles) to increase trapped air (thickness) for more insulation.

So the main idea is that feathers insulate by trapping air (using their structure) and have some insulating properties. So the best option is C: "feathers keep in heat with their insulating properties and by trapping air."

But wait, let's check the options again:

A: About feathers' origin (flight) – no.

B: About hollow tubes' properties – detail, not main idea.

C: About feathers keeping heat via insulating properties and trapping air – matches the main points.

D: About flight – no.

So the correct answer is C? Wait, but maybe I was wrong earlier. Let's confirm. The paragraph's main idea is feathers' role in insulation (trapping air and having insulating properties) to keep heat in. So option C is correct.
# Answer:
C. feathers keep in heat with their insulating properties and by trapping air.

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