题目内容：
回答题Scientists in the UK have announced that the piranhafish's reputation 21 a fearsome 22may well not be deserved．The fish, which is found in the Amazon in Brazil, have been 23as 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 25against
other predators, according to the team．"Previously it was thought piranhas shoaled as it26them to form a cooperative hunting group," said Professor AnneMagurran．" 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 29being killed．Piranhas of 30age stay in the middle of the group for 31and the 32 of the shoal differs according to the level of 33When 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．
A．as
B．as if
C．as though
D．since
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