Here we have a strengthen with necessary premise question. Remember that a premise is necessary for a conclusion if the falsity of the premise guarantees or brings about the falsity of the conclusion. First we check to see if the answer choice strengthens the passage, and then, if it doesstrengthen, we negate the answer choice to see if its negation makes the argument fall apart. If the answer choice does both those things then it is our correct answer.
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 the conclusion of the argument is 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.
(C): "There was no sudden mutation in the distemper virus which would have allowed the virus successfully to attack healthy North Sea seals by May 1988.
Does answer choice (C) strengthen the argument? Yes. The conclusion is that it was not the distemper virus alone that killed two-thirds of the seal population. Answer choice (C) helps strengthen the idea that nothing changed with the virus that could have made it sufficient on its own to be able to kill such a large portion of the seal population.
Negation: There was a sudden mutation in the distemper virus, which allowed the virus successfully to attack the healthy North Sea seals by May 1988.
This makes the argument fall apart because the explanation in the argument can, therefore, "rest there," since the mutation in the virus could have allowed it to "single-handedly" cause the large death in the seal population.
Hope that clears things up! Please let us know if you have any other questions.
