TOEFL Reading: ETS-TOEFL阅读机经 - 1JWCRQ756F6WK63BL$

In paragraph 3, why does the author mention distemper and its effect on seals? A. To explain why sociality is now relatively rare in seals B. To prove that parasites prevent animals from growing healthy and strong C. To show how easily social animals transmit potentially deadly diseases D. To provide an example of a species-specific disease that affects solitary animals