根据语境 ,填入适当的词语或句子

Mary: Good morning, Jim.

Jim: Good morning, Mary.  Look, there is (1)__________ over there.

Mary: Oh, it's a French book.

Jim: (2)__________?

Mary: I think it could be Tony's. He loves French very much.

Jim: No, (3)__________. He has been away from school for several days.

Mary: Is it Bob's?

Jim: (4)__________.You know he doesn't like French at all.

Mary: Oh, I know. (5)__________. Look, it has her name on it.

Jim: I think you are right. Let's give it back to Jane.

Mary: OK. Let's go.

收发信机中应设有收信电平监视回路，当正常收信电平低于（）时，便发出告警信号。

A、3db

B、8db

C、12db