问题补充:
While the book on the top shelf , you need to be very careful .
A. to reach
B. reaching
C. reach
D. are reaching
答案:
A. to reach
(答案→)B. reaching
C. reach
D. are reaching
解析:考查省略式,因为英语中如果主从句的主语一致的话,可以省略从句的主语和be动词,while reaching=when (you are) reaching ,句意是:当你够架子上的书时,你需要小心。选B。