This might seem unrelated to the argument, but it actually supports it, even if weakly. This points out another phenomenon that is what we would expect based on the author's hypothesis. If the cause of the seal die-off really is "the severe pollution of the North Sea", it would be strange for the pollution to affect only these seals. We would expect the pollution to affect other North Sea wildlife populations as well. This answer choice confirms that expectation: other creatures have also experienced unusual population drops in recent years.

You might be tempted to object that we don't know any details about those other population drops, so we can't assume they were caused by pollution. This is a fair point — but remember that no other causal hypothesis is offered in the stimulus or in this answer choice. As far as we're concerned, unless an answer choice offers an alternative, the only hypothesis on the table is North Sea pollution. Since this answer choice doesn't offer an alternative hypothesis and actually fits with what we would expect from the pollution hypothesis, we can say it supports the pollution hypothesis.

Irrelevant. Even if some countries have been "taking the lead" in trying to reduce pollution, we don't know what kind of an effect that has had. We have no idea what the actual level of pollution in the North Sea is, besides the stimulus telling us that it is "severe". So this answer choice doesn't help us figure out either way if North Sea pollution is the cause of the seal die-off, or if there is an alternative explanation.

Irrelevant. People fishing from the North Sea for "many years" isn't clearly connected to whether North Sea pollution is the cause of the seal die-off, or if there is an alternative explanation.

Irrelevant. Simply knowing that there are two species of seal in the North Sea doesn't help us determine whether the author's hypothesis is correct. It might help if we knew more about what was happening with each species: for instance, if one species of seal didn't experience a drastic die-off, while the other one did, or if both species were affected to the same extent. But on its own, this fact doesn't support the argument.

This is a tricky answer choice. You might have picked it because you thought it fit well with the author's hypothesis. If, first, the North Sea pollution weakened the seals' immune system, and then a brand-new disease came along, then it might seem to make even more sense that the disease was so devastating.

But it's really important to see that, far from supporting the author's hypothesis, this answer choice actually makes that hypothesis irrelevant. If this answer choice is true, then there's no need to imagine that North Sea pollution had to weaken the seals' immune systems before the virus could be so deadly. The virus would have been deadly even if the pollution didn't weaken the seals' immune systems, because the seals' immune systems (even at their strongest) had never been exposed to this disease before. Thus, this answer choice actually weakens the argument by presenting an alternative explanation for why the virus was so deadly.