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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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E is 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.
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