高中语法选择,请教语法好的同学：They have been in the library since (

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高中语法选择,请教语法好的同学：They have been in the library since (
A 8 o'clock
B it had been open
答案A我可以理解,但是我不知道为啥B是错的,为什么since引导的从句这里不能用过去完成时,语法上我觉得没有问题啊,图书馆开放这一动作就是在他们到图书馆之前发生的啊,怎么不能选B用过去完成时呢,

我们按步分析：
句子结构是：
主句+时间状语从句

主句They have been in the library-----完成时

时间状语从句：since_______.------表自从某个"时间点"以来

主句用完成时,since从句中应该+过去时间点（而非时间段）或一个短暂性动词.
B答案中,been+open（adj）表示一个持续性的状态,（理应是个短暂的动作或时间点即可）
A答案,8 o'clock是个时间点,符合要求.

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