21. Generally speaking, in the United States ____ 15-percent tip is normal for ____ most restaurants.

A. a; /                         B. an; the                    C. /; a                         D. /; the

22. Between the two periods of classes is a break,   we can have a short rest.

A. then                     B. where                   C. when                   D. which

23. There is no point ____ further. I think he has made a decision.

A. to argue                B. argued                  C. in arguing             D. argues