How to decline an offer politely?

[Schwein]

I am very lucky that I got accepted into two universities I applied. I have made my decision to attend one of the universities (let's call it university A), and I am thinking about declining the offer from the other university (university B). Should I tell the DGS of university B exactly that I am going to university A? Or should I just tell her I am going to another university? Besides, two professors from university B have exchanged email with me to help me know more about their program. Should I also tell these two professors I am not attending?

[ATMIT]

Hi,

If you had just asked about the program, you don't have email the professors. But if you had contacted them about their research work and projects, It would be courteous to email them that you had found a project in university A, which closely matches your interests.

You don't have to inform the graduate office of admission. If there is an online portal please decline the offer of admission there. If a reason is asked, mention that you received an offer from another program which closely matches your interests.