Question

read the passage.
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, but the iron in animal foods is more accessible to the body.
plant and animal food sources both contain iron, which helps red blood cells carry oxygen throughout the body.
heme iron, found in meat and fish, is easily absorbed by the human body.

Explanation:

The passage first states that the human body needs iron. Then it mentions plant and animal food sources of iron. It further explains that non - heme iron (from plants) is not easily absorbed (e.g., spinach example) and heme iron (from animals) is better utilized as it is in a protein ring for easy absorption. The main idea should cover both plant and animal sources and the difference in absorption.
The second option only mentions that iron helps red blood cells carry oxygen, which is a detail (the first sentence of the passage) but not the main idea. The third option only focuses on heme iron (animal source) and misses the plant - source comparison. The first option correctly compares plant and animal food sources (both have iron) and highlights that animal - source iron (heme iron) is more accessible (better absorbed) by the body.

Answer:

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