has long appeared that it was immune from
has long appeared to have been immune from
has long appeared as being immune to
had long appeared immune to
had long appeared that it was immune to
OA
没疑问，就想讨论一下关于E ：
没疑问，就想讨论一下关于E ：Ozone, a special form of oxygen that screens out harmful ultraviolet rays, reaches high concentrations twelve miles above Earth, where it had long appeared that it was immune to human influence; we have now realized, though, that emissions of industrial chlorofluorocarbons deplete the ozone layer.
这里第二个it指代的是Ozone，第一个it在这里算什么用法？这两个it在这里感觉很奇怪，但具体怎么个奇怪法，求指导