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
选D ,为什么用过去完成时？还有A的第一个it 到底指代哪一个？不是应该就近指代earth?为什么有人说指代ozone ?作者: susander 时间: 2010-6-25 20:40
