问题描述：
A ufo flew over my head while I was waking toward the bus stop yesterday
A UFO flew over my head while I was waking toward the bus stop yesterday 为什么是 A UFO flew ,不是 A UFO was flying呢?while引导的时间状语从句不是主从句都用进行时吗?怎么这里不是?
A UFO flew over my head while I was waking toward the bus stop yesterday 为什么是 A UFO flew ,不是 A UFO was flying呢?while引导的时间状语从句不是主从句都用进行时吗?怎么这里不是?
问题解答：
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