如何有礼貌的回复教授邀请加入的邮件？

如题，前两天收到0ffer通知，还不知道导师怎么选呢，今天居然收到了USC里面最心仪的教授邀请加入他们组的邮件，说给的奖还是USC最多的，让人不忍拒绝。。不过由于我申了十几所学校，目前结果只出了3个，还想再等等再做决定，不知道该如何回复？官方邮件的截止日期是4.15还早，但我害怕如果晚些做决定会不会就选不上这个心仪的教授了？或者措辞不当他又不想要我了额。。。实在没有经验，每次连给小蜜发邮件都要修改半天，求助各位大神啊！

Dear Prof. XXX,
Thank you for your kind invitation to your group. I am very interested in your research and USC is among my top choices. However, I still have some pending applications, so I am afraid that I could not make firm decision to attend USC at this point. Your understanding of my situation will be highly appreciated. I will tell you if I will join as soon as possible!

Thank you again.

Best Regards,

YYY

这种纠结事儿是越来越多了。。。