问题
选择题
The boy was so tired that he fell asleep ______ his mother could take off his clothes．
A．before
B．after
C．while
D．when
答案
答案：A
句意：这个小男孩如此的累以至于他的母亲还没来的及给他脱衣服他就睡着了。before 作为还没来得及之意时，常与情态动词could 连用。故选A。
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