GMAT Sentence Correction (SC) | Re: In the mid-1970s, since birds were overcome by pollution,

OA=D

I think there is a point in this question. Somehow, I notice that in GMAT, when something is said in generalization, you can use present form even though the time line is in the past. General statement here is (birds overcome) and it is fine in the present form. I have seen some questions similar to this one as well. Please correct me if I am wrong.