Ozone, a special form of oxygen that screens out harmful UV 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 chloroflorocarbons 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
My choice is B but the correct choice is D. Please explain.
I chose B because the NON underlined portion is in present tense as in "reaches high con..." although the second half of the sentence does have the past tense "realized".