How to ask for an extension on an acceptance offer

Hello, I was wondering if anyone has advice on how to write back a program (Washu Amgen) and ask for a slight extension of their acceptance deadline (March 6th) so I can hear back from other programs and make an informed decision. I'm really excited about the program but I'm just having a hard time committing when I'm still waiting to hear from 8 of the 15 programs I applied to. Does anyone have advice on how to structure this email? I don't want to upset anyone or give off a bad vibe.