题目选项分析

Although ice particles in the upper atmosphere benefit Earth in that they reflect and absorb solar radiation, they act as a global thermostat that thus keeps Earth either from burning up or freezing over, even though it also accelerates the destruction of the ozone layer by reacting with chlorofluorocarbons (CFC's).

Although ice particles in the upper atmosphere benefit Earth in that they reflect and absorb solar radiation, acting as a global thermostat and thus keeping Earth from either burning up or freezing over, they also accelerate the destruction of the ozone layer by reacting with chlorofluorocarbons (CFC's).