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  same as A

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

此题选D，可是这个谓语是什么，如果是FELL ，不是改变了句子的逻辑意思，因为句子的中心谓语应该是PROMPT，看来E合适点。不明白，看了以前的讨论，没有详细的解释，请指教！！！

这个是很明显的doing用来表结果的用法。

非常有效的表达方式，因为which和that都不能用来取代整句，而doing可以用来代表前面整句话，所以句子的重点不是如你所了解，是prompt的部分，而是fell的部分，陈述了一个事实。

如果你仍然困惑，别着急，先放一放，后面你会遇到很多这样的句子。