UMD发邮件问是否接受phd offer

据我所知UMD是415协议的成员啊。而且它家的offer上写着4.2号之前回复即可。
今天我收到邮件问我是否接受offer。尽管UMD我也很喜欢，但这毕竟才是第一所啊。请问我该如何回复啊。邮件如下：
Hope all is well.

In early January, I sent a letter notifying you of your application status for the Ph.D. program. This is just a follow up to see if you have received the letter.

Thank you,

非常感谢！