Question ID:PT8 S4 Q10 Passage:A distemper virus has caused two-thirds of the seal population in the North Sea to die since May 1988. The explanation for the deaths cannot rest here, however. There must be a reason the normally latent virus could prevail so suddenly: clearly 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. Stem:The argument concerning the immune system of the seals presupposes which one of the following? Correct Answer Choice:CChoice A:There has been a gradual decline in the seal population of the North Sea during the past two centuries. Choice B:No further sources of pollution have been added since May 1988 to the already existing sources of pollution in the North Sea. Choice 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. Choice D:Pollution in the North Sea is no greater than pollution in the Mediterranean Sea, off the coast of North America, or in the Sea of Japan. Choice E:Some species that provide food for the seals have nearly become extinct as a result of the pollution.