The stimulus tells us about seals that died from viral disease in 1988 in the Baltic Sea and off the coast of Scotland. There's a lot of information being thrown at us, so let's break down what we know:

(1) the death rate from the virus for Baltic seals was twice the death rate for Scottish seals; (2) Baltic seals had significantly more pollutants in their blood than Scottish seals; (3) pollutants make marine mammals more vulnerable to viruses.

From all this, the author concludes that pollutants were the likely cause of the Baltic seals' higher death rate. This argument follows a phenomenon-hypothesis structure. The conclusion is a hypothesis meant to explain the phenomenon of the differing death rates between two seal populations.

There are a few ways we can strengthen a phenomenon-hypothesis argument. One strategy is to eliminate alternative explanations to make the author's hypothesis more likely. For instance, we could say that the same virus affected both groups of seals. Another strategy is to identify corroborating evidence which we would expect to see if the hypothesis were true. For example, if viral death rates in other groups of seals were also correlated with higher pollutant levels, that would be consistent with the hypothesis.