答案：b
解析：主句谓语动词强调动作的迟缓性，只用before,这时常伴有时间段状语或时间段暗示。
eg.
It was quite a long time before he found the elephant.过了很长时间，他才找到大象。
We had walked a long way before we found some water.走了很长的路我们才找到水。
We had sailed for two days before we saw the land.航行了两天我们才发现陆地。
It was midnight before my brother came home.我哥哥直到半夜才回来。
before
有不明白的地方再问哟，祝你学习进步，更上一层楼！ (*^__^*)
选B
a
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