(2013·安徽高考)It be the vocabulary that caused you the problem in the exercise because you know a lot of words.
A. may
B. couldn't
C. should
D. needn't
网友回答
A. may
(答案→)B. couldn't
C. should
D. needn't
解析：选B 考查情态动词的用法。句意：你难以完成那道练习题不可能是词汇量的原因，因为你掌握了大量的词汇。根据原因状语从句because you know a lot of words可知，主句应该是否定的，A项和C项可排除；needn't表示“不必”，不符合题意。此处表示可能性，故选B。