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