3. B

at what time=when，引导宾语从句，when在从句中充当时间状语。故选B。

2. B

　when引导表语从句，表示时间。故选B。

1．A

本题含有强调句型，若把It is···that去掉，则很容易地看出what引导的宾语从句，what在从句中充当主语。故选A。

40.　 ______ he met with the difficulty did he realize the importance of our help.

　 A. Even though　　 　B. Never until　　　 C. Only before　　　 D. Ever since

[答案与解析]

39. The women carrying babies , come in first ,　　　 　?

　　 A．will you 　　　　　　　 B．will they 　　　　　　 C．do you 　　　　　　　 D．don’t you

38．In no country other than Britain, it has been said, ____ experience four seasons in the 　course of a single day.

A. people can　　　　　 B. you can　　　 C. can it　　　　　　　　　　 D. can one

37. They have no idea at all ____.

A. what he is working on　　　　　　 B. what is he working on

C. what did he work on　　　　　　 D. what he is working

36. 　　　　on the top of the mountain is an ancient tower dating back to hundreds of years ago .

　　　 A．To stand 　　　　　　 B．Having stood 　　 C．Standing 　　　　　　 D．Stand

35. No sooner _____ themselves in their seats in the theatre _______ the curtain went up.

　　　 A．they have settled; before　　　　　　　　　　　 B．had they settled; than

　　　 C．have they settled; when 　　　　　　　　　　　 D．they had settled;than

34. Only when 　　　　.

　　　 A．did he arrive the meeting began 　　　　　 B．he arrived did the meeting begin

　　　 C．did he arrive the meeting begin 　　　　　 D．he arrived the meeting began