问题补充:
The zoo keepers is worried because the number of visitors smaller
and smaller.
A. become
B. became
C. is becoming
D. have become
答案:
A. become
B. became
(答案→)C. is becoming
D. have become
解析:考查主谓一致。the number of……的数目,谓语动词用单数形式。根据句意用现在进行时。句意:动物管理员担心游客的数量正在变得越来越少。