RREP 2-8, 怎么区分过去完成时,和过去时+系动词

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

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这题选D, 可以为什么要用过去时? 另外, 过去时和过去完成时在这里是怎样区分的呀?