现在进行时由助动词be的现在时加上动词的现在分词构成,它的基本用法是表示现在正在进行的动作,所配的时间状语是now、at present、at this moment、these days等.如：
What are you doing now,John?
Where are you going?
Our team is leading.
The telephone is ringing.
George is translating a book now.（动作在包括现在在内的一段时间当中进行）
I'm not doing anything at present.
We are trying to plan our future.
We are eating more meat now.
He is speaking English much more fluently than he used to.
你给出的句子,题干中看到现在分词,现在分词前面当然配助动词be,构成现在进行时,排除A和D,Mary和Kate是两个人,是复数,选择C,表示“玛丽和凯特在那里吃什么?”希望我能帮助你解疑释惑.