Question

two kinds of iron
the human body needs iron to produce hemoglobin, a substance that helps red blood cells transport oxygen. without enough iron, a person may feel tired and weak. dietary sources of iron include plant sources, such as vegetables and beans, and animal foods, including poultry, red meat, and fish. however, plant and animal sources are not equal. the iron in plants, called nonheme iron, is not easily absorbed by the human digestive system. for example, a half cup of cooked spinach contains a healthy dose of over three milligrams of iron. unfortunately, only five percent of that iron can be used by the body. on the other hand, the iron in animal foods, called heme iron, is better utilized. heme iron is contained within a ring of protein, which helps the body to absorb it easily.
what is the main, or central, idea of the passage?
plant and animal food sources both contain iron, which helps red blood cells carry oxygen throughout the body.
plant and animal food sources both contain iron, but the iron in animal foods is more accessible to the body.
heme iron, found in meat and fish, is easily absorbed by the human body.

Explanation:

The passage first introduces that the human body needs iron for hemoglobin (which helps red - blood cells transport oxygen). Then it mentions plant (nonheme iron) and animal (heme iron) sources of iron. It further elaborates that nonheme iron (from plants) is not easily absorbed (e.g., only 5% of spinach's iron is usable) while heme iron (from animals) is better utilized as it is in a protein ring that aids absorption. So the main idea should cover both plant and animal sources and the difference in absorption.
The first option only mentions that both contain iron for oxygen transport but misses the key difference in absorption. The third option only focuses on heme iron (animal source) and doesn't mention plant - source iron. The second option correctly states that both plant and animal food sources contain iron and highlights the difference (animal - source iron is more accessible), which aligns with the passage's content.

Answer:

Plant and animal food sources both contain iron, but the iron in animal foods is more accessible to the body.