He was riding to school it began to rain.A. asB. soC. whileD. when

问题补充：

He was riding to school it began to rain.

A. as

B. so

C. while

D. when

答案：

A. as

B. so

C. while

(答案→)D. when

解析：考察连词。as 作为，随着；so 因此；while当……时，强调动作同时发生。 When当……时。句意：当天开始下雨时，他正在骑车去学校。结合语境可知选D。