PREP语法

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。说DO无法指虚词，但不知有什么更好的解释呢。 我只是觉得有DO语义表达更清楚。

白菜白菜

我也不懂~~~我觉得D也没错 而且不是补出谓语更好吗