Questions 76－81 are based on the following passage.

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.

76. Which of the following people could tell the difference between a cultured pearl and an organic pearl?

A. Scuba diver.

B. Fisherman.

C. Jeweler.

D. Clerk.

77. What is the chief component of nacre?

A. Sand.

B. Bead.

C. Calcium carbonate.

D. Biwa.

78. The difference between a pearl and a cultured pearl is the nature of the_____.

A. color

B. introduction of the irritating impurity

C. coating material

D. irritating impurity

79. Nacre is a substance that is______.

A. mechanically manufactured

B. the result of laboratory testing

C. organically secreted by the mollusk

D. present in the chemical composition of freshwater pounds

80. 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

81. Cultured pearl is formed by_____.

A. insertion of a pearl into a live mollusk

B. an oyster into which a piece of grit has been placed

C. putting in a live mollusk

D. placing a bead into culture