请教Prep-2-8

8.    (24773-!-item-!-188;#058&001691)

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

1、为什么用had过去完成时呢？

2、immune的固定用法是immune to是吗？

谢谢！

http://forum.chasedream.com/dispbbs.asp?BoardID=23&replyID=2225913&id=237175&skin=0

However, there is no clear explaination for the tense