We were going out for a walk it began to rain.A. whileB. beforeC. asD. when

问题补充：

We were going out for a walk it began to rain.

A. while

B. before

C. as

D. when

答案：

A. while

B. before

C. as

(答案→)D. when

解析：考查固定句型。固定句型：Be doing sth when…正要做某事，在这时（发生意料之外的事情）。When在…那时（并列连词）。句意;我们正要出去散步，在这时开始下雨了。故D正确。