It is difficult to predict when a slot machine will payoff. Sometimes it takes 200 pulls, sometimes 500, sometimes 1,000 or more. Since you know it must payoff sometime, you keep plugging in the coins. Slot machines like this are set up on:

A. Fixed-ratio schedule of reinforcement
B. Variable-ratio schedule of reinforcement
C. Fixed-interval schedule of reinforcement
D. Variable-interval schedule of reinforcement

My first answer was d but this was wrong. Is the correct answer b?