义齿基托采用湿磨法的目的是 （）

A.保护卡环，防止卡环折断

B.保持义齿的吸水膨胀

C.使基托厚度一致

D.避免摩擦产热使塑料焦化

E.抛亮基托

According to the author, swans in captivity are ______.

A．happy unless their wings have been cut

B．happy most of the time, but unhappy sometimes

C．unhappy most of the time

D．only happy when they are bringing up families