Most Americans would prefer to keep their problems ________ themselves, and solve their problems ________ themselves.

A．to; by

B．by; to

C．for; to

D．in; on

污水生化处理过程中产生的生物泡沫主要是由（）科的微生物所引起的。

A.放线菌

B.病毒

C.酵母菌

D.霉菌