题目选项分析

Ozone, a special form of oxygen that screens out harmful ultraviolet rays, reaches high concentrations twelve miles above Earth, where it has long appeared to have been immune from human influence; we have now realized, though, that emissions of industrial chlorofluorocarbons deplete the ozone layer.

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