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题库 / Prep2007E2-SC-8
Ozone, a special form of oxygen that screens out harmful ultraviolet rays, reaches high concentrations twelve miles above Earth, where it has long appeared that it was immune from human influence; we have now realized, though, that emissions of industrial chlorofluorocarbons deplete the ozone layer.
has long appeared that it was immune from
has long appeared to have been immune from
has long appeared as being immune to
had long appeared immune to
had long appeared that it was immune to
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调研了下，总结
1. 时态上，AE都有错，主句has／had appeared，后面从句was，就是错误的。这里从句应该与主语时态是一致的。
2 在we have now realized已经是现在完成，意味着在过去某个时间点之前我们都以为ozone是 immune from human influence，自然应该用过去完成时态。
3. it appeared后面加从句it blah blah是redundant，appear作为系动词，后面可以直接加形容词做表语。这样最为简洁清楚。
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