Well, these questions comes very frequently. The way you answer is
A) Blah Blah Blah
B) Unique answer that really applies to you.
For example. Let say it's google. I'll say that I can contribute XYZ because I have done it before. Also, I like the culture of Google where I can move into different project to expand my skillsets and I don't see other better company for this kind of opportunity.
I'm not say you should use my wordings but try to really relate with you in particular and be unique so that they'll remember you.