University I rejected is asking which other one I chose over them; should I tell? I got PhD admission offers from 2 Universities (say A and B) and yesterday decided to accept one of them (University A).
Today I decided to  let University B know that I would not be able to join them. After they acknowledged my refusal to join, they asked for the name of University A since they keep track of the same for statistical purposes.

Is there any harm in letting them know? Or is it better to keep it private?

Edit

I guess I was fretting over nothing. I sent B a brief reply saying that I was going to accept the offer from A (I've already signed documents with A so I'm reasonably confident they won't leave me in the lurch). B replied wishing me the best of luck at A. 

Thanks for the comments! I wasn't sure this was a common thing to ask for universities.
I can't think of any harm in telling them, and in the medium term, this isn't a secret, because it will certainly be clear by the fall where you've gone since your name will probably be put on the department website.  If you're uncomfortable, it's certainly fine to wait a bit until you've finalized things at the school you accepted; you don't even need to tell them you're doing that---just wait a bit before replying to the request.

It may help to know that it's standard for schools to want to know where students who reject them are going, so there's nothing unusual or suspicion raising about the question.  It's useful information because it helps them assess their program and admissions practices.