"What will you be doing in 5 years?" IV question

How do you address this Q in an interview? I was asked this yesterday and I think i messed it up.

I know most of u would say: say what u think you will be doing!
But in all honesty I am not sure.

But saying that in an interview sounds like I have given no thought to my future: its just that I cannot decide if i want to go for an MSN in a couple years time or not.

Is there a problem in saying that in an IV ?
Will they reject you if u say u want to move on because:
a. you might want the position to factor in that you will probably ask for help paying for the MSN?
b. you will probably not stay in the position once u have yr MSN ?

And if i decide not to go for an MSN , how can I phrase an answer which doesn't make it sound like i have no future plans?!!!

Thanks! please do reply! Specially someone who has had this asked in an IV!!