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

问题补充：

We were about to go 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：考查固定结构。Be about to do…when…，正要做某事，这时……。句意：我们正要去散步这时下起雨来。选D。