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, 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 compares plant and animal food sources of iron, highlighting that animal - based heme iron is more accessible (better utilized) by the body than plant - based nonheme iron. The first option about oxygen transport is not the main idea. The second option about iron helping red blood cells carry oxygen is not the focus. The third option accurately captures the central idea of the passage which is the comparison of iron accessibility from plant and animal food sources.

Answer:

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