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.
(A) has long appeared that it was immune from
(B) has long appeared to have been immune from
(C) has long appeared as being immune to
(D) had long appeared immune to
(E) had long appeared that it was immune to
(2)不与时间状语连接，用have done表示过去是这样，现在不是了；
而这个句子的逻辑含义就是过去很长一段时间里，臭氧都显得与人们的影响没有关系，但是现在人们已经意识到，工业二氧化碳的排放会怕破坏臭氧层；
也就是说，句子本身正是想表达出这种转折关系，为什么不用现在完成时，而用过去完成时；
只有一种解释：have done针对do时，表达出这种转折；而只有had done针对have done时，才能表达出这种转折；当然这需要讨论！
句子结构：Ozone, …, reaches high concentrations … above Earth, where... ; we have now realized that… 分号隔开意思并列的两个句子。第一个句子主语是oxygen由that引导的定语从句修饰，谓语是 reaches，where引导定语从句修饰Earth，该从句是主系表结构，appear作系动词，immune to human influence是表语。
（1）动词形式: 动词形式-过去完成时，很多情况下，过去时间由上下文表示出来，不需要表示过去时间的状语。
(2)指代一致，形式主语it指代了句子的真正主语（真正的主语在句子后部），因为有所指，所以句子中如果再出现其它有所指的it，就使it指代不一致，是错误的用法。
（a）it 引导的形式主语的句子，后面就不能出现it了，因为肯定会出现it的指代不一致；
（b）it引导的强调句式，后面可以出现it,因为强调句式中it是没有指代物的；
(A) it has long appeard that中it是形式主语，后面的it是指Ozone，两个it指代对象不一致。
(B) appeared to have been immume中的现在完成时多余，因为前面has appeared已经表达了完成时态。
(D) 正确，appear用作系动词，immune是adj.，简洁明了。分号后面是have now realized，强调now， 用了现在完成时，分号前面的long强调与现在的对比，用过去的时态较合理。
(1)immune to: not affected by sth 不受影响
You will eventually become immune to criticism. 你终究会变得不在乎批评。"
(2)immune from: protected from sth and therefore able to avoid it 受保护；豁免。=exempt Not even the President's wife was immune from criticism by the press. 甚至总统夫人都未能免遭新" 闻界批评。
appear 的用法作系动词, =seem, +adj. 作表语。作动词,
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发表于 2011-7-14 17:04:36
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