A keynote speaker was known for his many speaking engagements, but now he has limited time and a rational mind. Even he would eventually start to turn down speaking engagement when, by his judgement:

a. the total enjoyment (benefit) of the speaking engagement was maximized.

b. the additional enjoyment of one more speaking engagement (the marginal benefit) is rising.

c. the marginal cost is greater than the marginal benefit of an speaking engagement.