问题补充:
The zoo keeper is worried because the number of visitors smaller and smaller .
A. is becoming
B. become
C. became
D. to become
答案:
(答案→)A. is becoming
B. become
C. became
D. to become
解析:动物园饲养员很担心,因为游客的数量越来越少。结合语境可知下文为原因状语从句,描述的是现在正在进行的动作,故用现在进行时态,选A。