问一道prep题~~

流浪猫猫2008

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 function for teeth.

A)…

D) the horny jaws function as teeth do

E) the horny jaws function as teeth

我选D怎么就错了,不是助动词要尽量补出么?

cccnnn

function as +n, 我还第一次听说function as 后面可以加SV，听听其他人的建议