TOEFL Reading: ETS-TOEFL阅读机经 - QS5JUH3YU51F34O5L$

According to paragraph 7, the fact that during the 2,000 hours of observation no sentinels were attacked by predators may show that A. sentinels are extremely effective in warning members of their group about dangers B. it is safer for an individual to stand guard than to be guarded from predation C. it is hard to determine how hard guard duty is even from 2,000 hours of observation D. sentinels can ensure that foraging group members stay within 5 meters of a burrow