一道有点歪的逻辑题 ，来自PREP，求教

西瓜2011

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.

	their horny jaws function for teeth
	its horny jaws function for teeth
	its horny jaws function as do teeth
	the horny jaws function as teeth do
	the horny jaws function as teeth 选E。请教大家

西瓜2011

额我脑乱了。是语法==！