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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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；虽然可以连接两个独立句，但含分号的句子依然是一个句子而不是两个。
The ozone had appeared immune, but now we have realized……。had表示在我们意识到某事之前，人们不是这么看它的。
seem, appear, look，feel, smell, sound, taste是系动词，可以替换成be动词，直接接形容词；同时，系动词没有被动式，不管人还是物发起都用主动式，如：it tastes good; it appears nice.
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