21. Why must you keep complaining __________?

A. all the time             B. at times  

C. at a time            D. at one time

22. __________ I am is concerned, the idea is wholly crazy.

A. As long as           B. As far as  

C. As soon as           D. As well as

23. It was not until mid-night __________ he went back home after the experiment.

A. when      B. /           C. that      D. before

24. My idea is __________ we should go to help him out at once.

A. what      B. it          C. which    D. that