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标题: PREP里的语法题 [打印本页]

作者: DDM 时间: 2006-3-12 23:09
标题: PREP里的语法题

Ozone, a special form of oxygen that screens out harmful ultraviolet rays, reaches high concentrations 12 miles above Earth, where it had long appeared immune to human influence; we have now realized , though, that emission of industrial chloreofluorecarbons deplete the ozone layer.

划线部分是正确的答案，但是我不明白为什么这里要用had？我选了用has的，而且一开始就把这个正确的给排除了。

作者: DDM 时间: 2006-3-13 16:25
没有人讨论啊，为什么要用过去时呢？

作者: 入画 时间: 2006-3-13 22:50
we have now realized 是不是代表过去就知道了延续到现在，因此 had long appeared immune to human influence只能是知道以前的动作。

作者: jandjshi 时间: 2006-3-14 04:47
在OG逻辑题12A里的一句.
The state has recently removed some other restrictions that had limited the advertising of legal services.
看来过去完成和现在完成可以连用, 过去完成表示发生在现在完成之前的事.

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