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，请问为什么用过去完成时呢？没发现过去的时间提示啊！！！作者: Lacus 时间: 2011-1-4 15:17
（臭氧）以前（=had）是不受人类影响的，如今我们已经意识到氟氯碳化物破坏臭氧。