Mark, a Salesman at Signature Inc

Mark, a salesman at Signature Inc., is on a 10-day vacation. His manager Gregory calls him in the evening and asks him to meet one of their clients for dinner. Though Mark is on vacation and is not obliged to meet the clients, Gregory urges him to go for the meeting. Which of the following statements should Mark use to refuse politely and yet sound assertive?

A) "I am not going to meet the client when I am on vacation; it's not part of my job."
B) "You are to be blamed for this poor scheduling, and I am not going to suffer for your mistakes."
C) "Since I am on vacation, I have taken up some important personal work and I am busy with that."
D) "Gregory, you had approved my vacation a week ago, if I am not wrong."