问题补充：
Mr. Smith couldn’t open the door because his naughty boy it from the inside.
A. would lock
B. was locking
C. has locked
D. had locked
答案：
A. would lock
B. was locking
C. has locked
(答案→)D. had locked
解析：句意：史密斯先生打不开门，因为他淘气的儿子从里面锁上了。表示发生在过去的过去，故用过去完成时，选D。
