GMATPREP3 - In the mid-1970's, since birds were overcome by

First, read the sentence for the primary meaning:
Main action: birds were falling
Effect: officials devised a plan

We want to find a sentence that makes this point succinctly, without any errors in logic or grammar.

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
- a prepositional phrase such as "in the 1970s" should be followed (except in rare cases) by an INDEPENDENT clause, not by a dependent clause such as "since..."
- "this" is unclear. What exactly is prompting officials?
- the present participle in "were... routinely falling" suggests that perhaps the same birds were falling again and again. Since each bird (presumably) fell only once, it's better to say "fell."

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
- same issue as in A with "In... since..."
- "it" is ambiguous here. The pronoun seems to be referring to "the whole situation mentioned in the previous clause." For cause-and-effect, better to make the "birds" clause independent, and the "officials" part either a dependent clause or modifying phrase.

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
- getting rid of "since" turns "birds" into the main subject - an improvement on A and B.
- however, there is a verb tense issue: "had been overcome" should only be used in reference to another past tense event that interrupted. "I had been taking vitamins for years until my doctor told me that they were useless." In this case, "had been overcome" does not make sense with the time marker "in the 1970s" or with the parallel action "and routinely fell."
- in the context of this sentence, if "devised a plan" is simple past tense, the reducing of air pollution happens long after that. So better to say "... that would reduce" or "... that aimed to reduce" rather than "that reduced." Putting both in the simple past tense does not create the necessary time distinction between action (plan) and effect (reduction).

D. birds overcome by pollution routinely fell from the sky above Los Angeles freeways, prompting officials in California to devise a plan to reduce
- logical independent clause ("birds fell") and modifying phrase ("prompting")
- "to reduce" makes more logical sense than "that reduced"

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
- "birds... were prompting officials" doesn't make such sense. "Birds fell..., prompting officials" is much more logical. The birds themselves don't prompt; the fact that they're falling does.

The answer is D.