If you've ever shopped for produce in a U. S. grocery store, you've probably seen yams and sweet potatoes on display. Generally, the ones labeled "yams" have reddish skin and orange flesh, while the sweet potatoes have gold-colored skin and white flesh. Yet scientifically speaking, those yams aren't yams at all; they're really a variety of sweet potato. True yams are native to Asia and Africa and belong to their own family of plants. They have rough, bark-like brown skin and dry, starchy flesh. True yams are not often stocked in U. S. grocery stores. Sweet potatoes, on the other hand, belong to the morning glory family of plants and are widely grown in the United States. Unlike true yams, which are tubers, or
underground stems, sweet potatoes are roots, like carrots.

What is the main idea of the passage?
A:True yams are grown in Asia and Africa and are not often sold in the United States.
B:True yams are tubers, or stems, while yams sold in U. S. grocery stores are roots.
C:True yams are starchy tubers from Asia and Africa, while U. S. yams are a type of sweet potato.