再问 gwd 10-20！！

Q20:
In the mid-1970’s, since birds were overcome by pollution, and routinely falling from the
sky above Los Angeles freeways, this prompted officials in California to devise a plan
that reduced automobile emissions.

A. since birds were overcome by pollution, and routinely falling from the sky above
Los Angeles freeways, this prompted officials in California to devise a plan that
reduced
B. since birds that had been overcome by pollution were routinely falling from the
sky above Los Angeles freeways, it prompted officials in California to devise a
plan that would reduce
C. birds had been overcome by pollution and routinely fell from the sky above Los
Angeles freeways, prompting officials in California to devise a plan that reduced
D. birds overcome by pollution routinely fell from the sky above Los Angeles
freeways, prompting officials in California to devise a plan to reduce
E. birds overcome by pollution and routinely falling from the sky above Los Angeles
freeways were prompting officials in California to devise a plan to reduce

答案是D, 我选的是B, 以前也有人讨论， 但是觉得不能说服，

我认为这个句子要表达出“因为...所以"的意思，D没有这个意思， 而且B也没有错阿？

it 不可以么？

可是选择D,改变了整个句子的重心，如此以来为什么不选择B呢，it 指代前面的内容为何又不可以呢？

open to discuss

 表伴随结果,整个句子是原因,导致分词动作产生,无逻辑主语。

In an effort to reduce the number of fires started by cigarettes, a major tobacco company is test-marketing a cigarette in which thin layers of extra paper are used to decrease the amount of oxygen entering the cigarette, thereby slowing the rate at which it burns and lowering the heat it generates.

A.      in which thin layers of extra paper are used to decrease the amount of oxygen entering the cigarette, thereby slowing

B.       in which they use thin layers of extra paper in decreasing the amount of oxygen entering the cigarette, which slows

C.      that uses thin layers of extra paper to decrease the amount of oxygen when it enters the cigarette, thereby slowing

D.      for which thin layers of extra paper are used to decrease the amount of oxygen that enters the cigarette, and thereby slowing

E.       using thin layers of extra paper in decreasing the amount of oxygen entering the cigarette, which slows

选a 也就是这个用法。。。记住就可以了，thereby也可以不加的。