You were invited to be a key speaker at a conference and gladly accepted the invitation. However, as the conference draws near, you realize you will not be able to have your presentation ready in time. You feel bad, but you won't be able to do it and so you want to let them know about it without making a fake excuse and you say:
A) I really must apologize for this but I've been asked to present at another conference on the same day. Sorry about that.
B) There's just no way that I will be able to speak at the conference because of my health problems. Can you get someone else?
C) I'm afraid you didn't give me enough time to prepare my presentation, so I'm going to drop out of the conference.
D) Look, I'm sorry but would it be OK with you if I don't present at the conference?
E) I realize that I'm letting you down, but unfortunately, my presentation will not be ready and I'm afraid I'll be unable to speak at the conference.