问题补充：
The little child usually felt frightened at night, so not until .
A. did he fall asleep his mother left the room
B. he fell asleep had his mother left the room
C. did he fall asleep did his mother leave the room
D. he fell asleep did his mother leave the room
答案：
A. did he fall asleep his mother left the room
B. he fell asleep had his mother left the room
C. did he fall asleep did his mother leave the room
(答案→)D. he fell asleep did his mother leave the room
解析： 考查倒装句，not until 放在句首时，从句不倒装，主句倒装，可排除A和C两项，B选项从句没倒装，主句倒装了，但是主语后的动词应用原形，所以不正确。故选D。
