Assumption: Pollution made seals vulnerable to distemper virus by weakening their immune system....E is the only option seems supporting the assumption/explanation to argument

B, give additional info and somehow out of scope. if those countries are reducing the pollution, then Northern Sea is not polluted to the extent that polution can weaken the immune sys of the fish.

I would go with E. Still i am not sure, on the exam day i would pick it anyway

Ok. I too got this CR wrong. But the one tip that I learned on research from a CR book is that for any "strengthen/most strongly supports" type of questions, analogies/similarities, though out of scope, are usually very possible answer choices. Here, choice A provides an analogy/similarity with the fact that:

A distemper virus has caused two-thirds of the seal population in the North Sea to die

Choice (A) says:
At various times during the last ten years, several species of shellfish and seabirds in the North Sea have experienced unprecedented steep drops in population

The remaining answer choices can be eliminated by POE. IMO, the choice A is justified by the above reasoning.

OA is A.

I was between A and B and I picked B.
I can't see the relations in A since you don't know why all other species died. It can be some other cause other than pollution. In B, I feel that it shows there is pollution on the North Sea.

IMO it's B, in A nothing relates "steep drops in population" to "the severe pollution..."

Likes ive been saying. Nothern Europe and Scandinavia have been taking the lead to prevent pollution from entering the North Sea at its source.

Meaning that pollution does not come from the North Sea or someone is not pollutiing directly into the North Sea. Also, these countries are pretty much preventing pollution from entering the North Sea. Even if these countries cannot prevent all the polution, there is NOTHING in this choice that relates pollution to decreased populations of Seals or any animal.

Just says that some pollution may or may not have entered the North Sea from a source other than the North Sea. You can't extend this argument to the main passage by saying this pollution must have been the cause. We need more info saying that it did have an effect.

After all this, even if u still can't get it, just look at the passage: "Severe pollution." There is no indication from answer choice B, that the pollution that may or may not enter the North Sea is "Severe." In fact choice B suggests the opposite. B/c these countries are preventing the pollution from its source, it is very likely that the pollution is NOT severe. maybe not true, but u can't assume from this that the pollution that does enter the North Sea to be SEVERE.

hope that helps.