Why couldn’t Betty join the dancing party?

M: We are going to have a dancing party tonight. Would you like to join us, Betty?
W: No, I’m afraid not. I have an appointment with my history professor tonight.

M: Come in please. Congratulations, Susan.
W: What?
M: After reviewing your record, the management has decided to send you abroad for training.
W: What a pleasant surprise, Mr. Asimov. What did I do to deserve that?