While         the book on the top shelf , you need to be very careful .
A．to reachB．reachingC．reachD．are reaching
B

試題分析：考查省略式，因為英語中如果主從句的主語一致的話，可以省略從句的主語和be動詞，while reaching="when" (you are) reaching ,句意是：當你夠架子上的書時，你需要小心。選B。
點評：省略式是重要的考點，要求一定是主從句主語一致的時候才可以，另外做題時，可以使用還原法，把句子完整的形式寫出來，再進行省略。
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