Answer:
A:True yams are grown in Asia and Africa and are not often sold in the United States.
This is correct because the author starts comparing the two yams at the end and we realize the U.S doesn't even have yams and they are mistaken. They really grow in Asia and Africa is what the main idea is.
B is not correct because it is just the description of the yam, but it wasn't the point of the passage. The author used this detail to get to the main idea.
C is not correct because of the misinformation. While the first half of the answer is true about Asia and Africa grown yams, it says then they are a sweet potato in the U.S. That's what they are said as in the U.S, but the point of the passage was to clear up what yams really are and where they originate.