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
In generally, use simple past tense if possible. For instance, you will say,
"Yesterday, I found out that Dinosaurs lived on Earth for 3 million years" (sorry that I made up the number....)
Use past perfect tense ONLY if you need to emphasis the sequence of the action.
"Yesterday, I found out that Dinosaurs had lived on Earth for 3 million years before they died out."
Here, you wanna to emphasize the sequence of two actions, namely "had lived" and "died out."
In the absence of other better choices, you may use "had".
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