A．For B． Though C．While D．If

17.You won't succeed_____harder.

A．unless you will work B．unless you work

C．unless you don't work D．if you won't work

18.I wonder if he_____us, and I think if he_____us we'll be able to complete the task ahead of time.

A．helps, helps B．will help, helps

C．helps, will help D．will help, will help

19.I don't like to be interrupted if I_____.

A．speak B．will speak C．am speaking D．spoke

20.If you_____ this experiment you will understand the theory better.

　A．will be doing B．have done C．will have done D． would do

21.I would like to do it_____I like it.

A．since B．because C．because of D．now that

22. _____everybody is here, Let's set off.

A．Since B．Because C．For D．After

23.It was_____ he was ill that he was absent yesterday.

A．because B．as C．since D．now that

24. _____it is raining, we had better take a taxi.

A．For B．As C．Because of D．When

25."Why can't you do it now?" "_____I'm too busy."

A．Since B．As C．Because D．For

26.He must have passed this way, _____here are his footprints.

A．since B．because of C．now that D．for

27. _____everybody is here, Let's begin our meeting.

A．Now that B．Because C．For D．When

28.His speech made_____deep impression on the audience that they could hardly forget it.

A．such a B．so a C．so D．such

29.They worked hard_____they finished their work ahead of time.