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 答案是D,如何理解啊？怎么可以用had啊？~ | 
