The speaker explains that the recent deaths of the seal population in the North Sea was not caused by the distemper virus alone, rather it was caused by the immune systems of the seals being weakened due to pollution in the North Sea waters, which made the seals more susceptible to the virus.
The virus is normally a latent virus. Thus, the speaker's hypothesis is that the severe pollution of the North Sea waters "must have weakened the immune system of the seals so that they could no longer withstand the virus."
So we are looking for the answer choice that strengthens the conclusion that it was not the virus alone, and, in fact, it was the pollution ALONG WITH the virus that caused the extreme death toll of the seal population in the North Sea waters.
Answer choice (A) states: "At various times during the last ten years, several species of shellfish and seabirds in the North Sea have experienced unprecedentedly steep drops in population."
Here we are given evidence of different species that share the same environment as the seals who have also had a severe population drop. This helps support the fact that it's not just the virus, but also possibly something in the environment that is exacerbating the situation for the animals in the North Sea. You should note that these seabirds and shellfish are specifically from the same areas as the seals. So, since these other animals who share the waters with the seals are also dying, it helps the conclusion that an environmental factor is causing (or is also causing) the deaths.
Answer choice (D) states: "There are two species of seal found throughout the North Sea area, the common seal and the gray seal."
This is irrelevant to the argument. It doesn't matter how many species of seal are found. The issue at hand is determining whether the drop in the North Sea seal population was caused only by the virus or by environmental factors that exacerbated the virus's effects.
Hope that helps! Please let us know if you have any other questions.
