While the book on the top shelf you need to be very careful .A. to reachB. reachingC. re

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。