TOEFL Reading: ETS-TOEFL阅读机经 - 351IATA1U4LX35L4L$

According to paragraph 6, bison in herds needed to be able to crop grass quickly because A. the amount of food available for an individual had decreased B. the bison body had become smaller C. the chances of being attacked by a predator had increased D. bison had grown a hump to support the head's weight