问题
选择题
This street is very crowded at this time of day.You have to be very cautious while ______.
A．to drive
B．being driven
C．driving
D．having driven
答案
C
题目分析：句意：街上这个时候很拥挤，开车的时候，你要小心。这里使用了时间状语从句的省略，完整的是：You have to be very cautious while you are driving.当主从句的主语一致的时候，可以省略从句的主语和be动词，所以选C。