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I was on my way to school this morning____it suddenly began to rain heavily.
A、before
B、since
C、when
D、after
【正确答案】：C
【题目解析】：句意：今天早上当我在上学的路上，突然下起了大雨。before 之前；since 自从；after 之后；when 当；因此C符合题意