某堤防工程发生质量问题后，以下（）处理方式是符合《堤防工程施工质量评定与验收规程（试行）》SL239-1999的。

A.全部返工重做，但只能评定为合格

B.可进行加固补强，经鉴定能达到设计要求的，可重新进行质量等级评定

C.对于Ⅰ级堤防，无论采用何种方式处理，其质量等级只能评定为合格

D.全部返工重做，可重新评定其质量等级

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