prep02 243 求解

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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).


(A) acting as a global thermostat and thus keeping Earth from either burning up or freezing over, they also accelerate


(E) they act as a global thermostat to thus keep Earth from either burning up or freezing over, but they also accelerate

我的问题很简单，就是E ； 很多解答都是E没有主句，因为 Although。。。。but。。。
为什么不能理解为Although。。。 they act as。。。。。。，but呢？这里they act 。。。 不就可以当主语了吗? 难道是因为主语中间，两头的从句导致了back to back modifier?
（E 的意思可能有些问题，因为alothough 后面应该给出一个相反的意思，大师they act明显是解释，没有转折。---这些先不讨论，我只是好奇为什么这个不能当主句在这里）十分感谢1!!!!!

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是因为太简单了所以没人爱搭理我吗。。。

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wowowowowowo来人呀