疑问,急~~

Q31:

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.

1. 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
2. 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
3. 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
4. birds overcome by pollution routinely fell from the sky above Los Angeles freeways, prompting officials in California to devise a plan to reduce

5. 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

D选项没有中心转移吗???原题重心在PROMPTED 上吧??而D项在BIRD FELL FROM 上

e项的结构一塌糊涂一团糟。

本题的中心是birds受难被officials重视，d项prompting 修饰了这个情况并introduce officals's plans，没有便宜