I use a clock to wake me up because at six o'clock each morning the train es by my house.(2012·新课标全国)
A. couldn't
B. mustn't
C. shouldn't
D. needn't
网友回答
A. couldn't
B. mustn't
C. shouldn't
(答案→)D. needn't
解析：句意：我不必用钟表唤醒自己，因为火车每天早上六点经过我的房子。根据原因状语从句的内容可知此处表示的是“不必”。A项表示没有能力，B项表示禁止，C项表示不应该，都与所给语境不符。