见过语法差的~~没见过像我语法那么差的~~急寻解释~~GWD8~~在线等

5.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
这题我读的很别扭，我明白是说鸟都死翘翘了，加州政府就起草计划减少污染。可是这个句子读着感觉有问题，所以就选了A，因为从逻辑看来，鸟的掉落是原因，计划为结果，那么句子的重心为计划。但答案为D，有nn能帮忙解释下这个句子的结构吗~~和为什么A是错的，D是对的~~？？