The boy was so tired that he fell asleep his mother could take off his clothes. A. before

问题补充：

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. before

B. after

C. while

D. when

解析：句意：这个男孩太累了，以至于在他的妈妈给他脱衣服之前，他就睡着了。Before在……之前 after在 …… 之后 while然而 when当…… 时候，选A。