we were reading books, while he knocked the door.
答案B.while
While Mary was writing a letter, the children were playing outside.
玛丽写信时,孩子们在外面玩耍.
I lived in a hostel while I was a student.
我求学期间住在青年招待所里.
Please keep the watch for me while I go swimming.
我去游泳,请代我保管这表.
He took advantage of her while she was drunk.
在她喝醉时,他诱奸了她.