单项选择题

Scientists in the UK have announced that the piranhafish’s reputation (21) a fearsome (22) may well not be deserved. The fish, which is found in the Amazon in Brazil, have been (23) as deadly carnivores (食肉动物) that work in shoals to overwhelm their prey and strip it of its flesh in seconds.
However, experts (24) St Andrews University say that piranhas are omnivores (杂食动物) that mainly eat fish, plants and insects. They form big groups not to hunt but to defend (25) against other predators, according to the team.
"Previously it was thought piranhas shoaled as it (26) them to form a cooperative hunting group," said Professor Anne Magurran. " (27) , we have found that it is primarily a defensive behavior."
Piranhas can be attacked by animals (28) dolphins, caimans and large fish, so forming a shoal is a good way of (29) being killed. Piranhas of (30) age stay in the middle of the group for (31) and the (32) of the shoal differs according to the level of (33) . When the water level is (34) , the fish form small groups as there is space to escape, but when the water level drops, they form large groups as protection (35) predators.

33()

点击查看答案&解析