TOEFL Reading: ETS-TOEFL阅读机经 - 2J1MPDR931IE41EPL$

According to paragraph 2, the broad-spectrum foraging argument holds that humans shifted from big-game hunting to a hunter-gatherer lifestyle primarily because A. they had begun developing more sedentary communities B. populations required a broader range of food sources C. it was easier and more effective to hunt smaller animals D. the very large Ice Age animals had begun to die out