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

边坡开挖中如遇地下水涌出，应先排水，后开挖。