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题库 / Prep2007E2-SC-43
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).
acting as a global thermostat and thus keeping Earth from either burning up or freezing over, they also accelerate
acting as a global thermostat and thus keeping Earth either from burning up or freezing over, while also accelerating
act as a global thermostat and thus keep Earth from either burning up or freezing over, while also accelerating
they act as a global thermostat that thus keeps Earth either from burning up or freezing over, even though it also accelerates
they act as a global thermostat to thus keep Earth from either burning up or freezing over, but they also accelerate
(A) 正确，原文的acting and keeping是作benefit的伴随状语，解释如何benefit Earth。Although ice particles benefit..., they also accelerate the destruction...，原文要表达这样一个对比：尽管有益，也有害。
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