You Need to Write a Message Refusing a Request to Act

You need to write a message refusing a request to act as master of ceremonies at a silent auction for a charitable cause. Which sentence would be most appropriate for the closing?

A) Please call or send me an e-mail if you have further questions.
B) Because you invited me on such short notice, I will not be available.
C) Once again, I am very sorry that I cannot assist with your silent auction.I apologize sincerely.
D) Please keep me in mind if I can assist with this worthwhile event next year.