The most popular organic gem is the pearl. A pearl is the response of a marine mollusk to the presence of an irritating impurity accidentally introduced into its body; a cultured pearl is the result of the intentional insertion of a mother-of-pearl bead into a live mollusk. Whether introduced accidentally or intentionally, the pearl-making process is the same: the mollusk coats the irritant with a substance called nacre. Nacre is composed chiefly of calcium carbonate. Because very few natural pearls are now on the market, most pearls used in fine jewelry are cultured. These include "Biwa" pearls and most other freshwater pearls. Cultured pearls are not easily distinguished from natural pearls except by an expert.
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
