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！！！ | 
