I was about to go out it began to rain.A. when　　　B. asC. since　　　D. while

问题补充：

I was about to go out it began to rain.

A. when

B. as

C. since

D. while

答案：

(答案→)A. when

B. as

C. since

D. while

解析：句意：当开始下雨的时候，我正要外出。When当……的时候 as作为 since自从 while然而，选A。