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。
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