Bill Has Voluntarily Organized Office Birthday Parties for the Past

Bill has voluntarily organized office birthday parties for the past 10 years because he enjoys making people happy. One day, Brenda, his boss, tells him that he's been doing such a great job that she's going to pay him an extra $200 next month. However, when Bill gets his check, it's the same amount that it has always been. Which of the following is correct?

A) Brenda's promise is enforceable because it is based on a bargained-for exchange.
B) Brenda's promise is unenforceable because it is based on past consideration.
C) Brenda's promise is enforceable because Bill performed a service for the company and should be paid even if it's not part of the job description.
D) Brenda's promise is unenforceable because the situation is governed under the preexisting duty rule.