We were about to go outing it began to snow heavily.A. whenB. whileC. asD. until

问题补充：

We were about to go outing it began to snow heavily.

A. when

B. while

C. as

D. until

答案：

(答案→)A. when

B. while

C. as

D. until

解析：考查固定搭配。Be about to do sth when…“刚要…这时”.。句意：我们刚要外出这时天开始下雪。