Maize contains the vitamin niacin, but not in a form the body can absorb. Pellagra is a disease that results from niacin deficiency. When maize was introduced into southern Europe from the Americas in the eighteenth century, it quickly became a dietary staple, and many Europeans who came to subsist primarily on maize developed pellagra. Pellagra was virtually unknown at that time in the Americas, however, even among people who subsisted primarily on maize.

Which of the following, if true, most helps to explain the contrasting incidence of pellagra described above?

1. Once introduced into southern Europe, maize became popular with landowners because of its high yields relative to other cereal crops.
2. Maize grown in the Americas contained more niacin than maize grown in Europe did.
3. Traditional ways of preparing maize in the Americas convert maize’s niacin into a nutritionally useful form.
4. In southern Europe many of the people who consumed maize also ate niacin-rich foods.
5. Before the discovery of pellagra’s link with niacin, it was widely believed that the disease was an infection that could be transmitted from person to person.

请问一下：为什么选C呢？