usc发邮件催offer该如何处理

一个星期前收到了usc的offer，一直没有admit想等别的学校的结果，结果今天早上收到了院长的邮件
I would like to personally congratulate you on your admission to the Viterbi School. You should be quite proud of this accomplishment.
...
I realize that you may have many options of universities for advancing your engineering education.
...
这个是啥意思，催一下我接offer嘛= =，其实比较想去usc的但在等umich，有点害怕它撤回offer之类的= =，所以应该如何处理呢？
此外，如果接了的话最后不去可以嘛？