PREP有一道难题不解？

Although the turtle has been toothless for more than 150 million years, in some contemporary turtle species the moderately sharp and jagged edges of their horny jaws fuction for teeth.

A. their horny jaws function for teeth

B. its horny jaws function for teeth

C. its horny jaws function as do teeth

D. the horny jaws function as teeth do

E. the horny jaws function as teeth

正确答案是E，我不明白，请高人帮我讲一讲，谢谢了

A. their horny jaws function for teeth

B. its horny jaws function for teeth

C. its horny jaws function as do teeth

D. the horny jaws function as teeth do

E. the horny jaws function as teeth

首先从排除A，应该用单数，其次是horny jaws 扮演牙的功能。FUCTION是动词 去除C/D    AS这里是作为牙，所以BE中选E