We were about to leave the classroom it began to rain.A. whileB. whenC. asD. before

问题补充：

We were about to leave the classroom it began to rain.

A. while

B. when

C. as

D. before

答案：

A. while

(答案→)B. when

C. as

D. before

解析：be about to do sth. when …正要干某事，这时…。