In botany, what distinguishes a yam from a sweet potato based on their biological classification and nutritional composition?
A) Yams are tubers, while sweet potatoes are rhizomes.
B) Yams are native to South America, while sweet potatoes originate from Africa.
C) Yams contain higher levels of starch, while sweet potatoes are richer in beta-carotene.
D) Yams are harvested year-round, whereas sweet potatoes are seasonal crops.