prep SC I had confusion on verb tense

[frankieguo]

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
when I do this question, I quickly ruled out D&E, because I think it makes no sense to use past perfect tense. And C has being in it and it's awkward. A is wrong with "it". So I chose C..
Could anyone told me why "had appeared" is right here?

[RonPurewal]

search the forum before posting, please.

ozone-reaches-high-concentrations-twelve-miles-t3584.html