I find this to be a pretty difficult question.

The reason being, it seems quite a few answers would make sense.

The conclusion is that air-pollution can be stopped by regulation and because of the regulation, the number of bird species will increase.

In my opinion A,B, and D all fit as an assumption here.

In A - For the regulations to work (and thus be the reason for the increased birds) the author must assume that all the pollution is coming from areas effected by the regulation. If the pollution is coming from elsewhere, then the regulation would have no effect, and thus, would not be the reason.

In B - The author assumes that these regulations are having a significant impact on the quality of the air in the area and this is the reason bird species have increased.

and in D - For the author to say all cities must impose regulations assumes that cities of course would like more birds around. Some cities may not like this (more bird poop?).

In a test situation, I probably would have gone with B. But I sure would have been stumped...

Any other ideas on this? Maybe a better way to solve this one?

The questions is asking about EXCEPTION..

IMO, all options can work as assumptions. However, A is too extreme by mentioning "almost entirely". That's the only basis for me to choose A

I am pretty sureit is A