PREP里的语法题

Ozone, a special form of oxygen that screens out harmful ultraviolet rays, reaches high concentrations 12 miles above Earth, where it had long appeared immune to human influence; we have now realized , though, that emission of industrial chloreofluorecarbons deplete the ozone layer.

划线部分是正确的答案，但是我不明白为什么这里要用had？ 我选了用has的，而且一开始就把这个正确的给排除了。