问题
单项选择题
The main idea in this passage is that ______
A．most marketable pearls are cultured because nature does not produce enough of its own to satisfy the market
B．cultured pearls are of a higher quality than natural pearls
C．there are two major methods of pearl-making
D．a natural "drought" of pearl production is taking place
答案
参考答案：C
解析：[分析] 主旨题型。 阅读全文可知本文介绍了珍珠有天然和人工养殖两种形成方式；所以选项C正确。