PREP 一道 非常奇怪的 时态题 NN们救命呀

呼啸700

8.(24773-!-item-!-188;#058&001691)

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(答案)

(E) had long appeared that it was immune to
完全不懂为什么是 过去完成时呀？？？？？？

junyihuo

如果用现在完成时，表示的意思就是，现在人们仍然认为臭氧是免于人类影响的。但是后面说了，这种看法是错的，臭氧还是会被人类影响。

所以一定要用过去完成时。

呼啸700

但是我有一点不明白 如果用过去完成时 那么 应该有一个一般现在时  作为基点呀 那个标准 在哪里呀 请NN 给我解释一下 谢谢你

cycyang

后面的分句“we have now realized, though, that emissions of industrial chlorofluorocarbons deplete the ozone layer.”应该就是过去完成时的基点，因为它表明目前已经意识到以前的观点是不正确的。第一个分句中“。。。immune from human influence”显然说的是有这种正确认识之前的事。

pansimply

我想问的是解释里面的形式主语问题。
我觉得A的where it has long appeared that it was immune from中第一个it是形式主语，沟通it has appeared that这样的形式，第二个it是指ozone。因为两个it指代不清，所以错。
但是D的where it had long appeared immune to中的it就是指ozone，应该不是形式主语。

不知道我说的对不对。