I _______ keep dogs for protection, but now I keep them as pets.

A．would rather

B．had better

C．used to

D．have to

如果噪声随时间变化时大时小，这种噪声称为（）.

A.机械性噪声

B.电磁型噪声

C.稳态噪声

D.脉冲噪声