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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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AE where后的第一个it指ozone，that后的it也指ozone，说了两遍主语 错
too wordy. "appeared immune" is just fine on its own. Notice that we have "it appeared immune." The "it" here refers to the ozone. Thus, to say: "it appeared that it was immune" would be unnecessary.
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
B tohavedone ，todo不定式的过去时，太冗长了，和前面的had appear重复，而且主要是前面的时态错
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