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 first mentions the body's need for iron. Then it compares plant - sourced nonheme iron (hard to absorb) and animal - sourced heme iron (easier to absorb). The main idea should cover both plant and animal sources and the absorption difference.
The first option "Plant and animal food sources both contain iron, but the iron in animal foods is more accessible to the body" is correct. The second option only mentions the function of iron (helping red blood cells carry oxygen) without the key comparison of absorption. The third option only focuses on heme iron (animal - sourced) and doesn't mention plant - sourced iron, so it's not comprehensive.

Answer:

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