He was riding to schoolhe was hit by a car this morning. A. whileB. whenC. asD. so

问题补充：

He was riding to schoolhe was hit by a car this morning.

A. while

B. when

C. as

D. so

答案：

A. while

(答案→)B. when

C. as

D. so

解析：考查时间状语从句的连接词。句意“今天早上，他正在骑车去上学的路上，这时突然被车撞了。”while前后都要用进行时；when“当；突然”，后跟长动作或短动作；as“一边......，一边......”，so“因此”。故选B。