Sam,a Used Car Salesman,doesn't Know How Many Customers He Will

Multiple Choice

Sam,a used car salesman,doesn't know how many customers he will have to approach until someone buys a car,but he does know it is possible that the very next customer will buy a car.Sam's sales behaviour is reinforced according to which type of reinforcement schedule?

A) variable-ratio
B) fixed-ratio
C) fixed-interval
D) variable-interval