A gambler at a slot machine and an office worker paid every second Friday are both on reinforcement schedules. Why is the gambler's behaviour so much harder to extinguish?

C. Because a variable ratio gives no way to tell a dry spell from the end.

Extinction requires detecting that reinforcement has stopped, and the schedule sets how detectable that is: twenty losing pulls is an ordinary run, and so is fifty. The evidence that would end the behaviour looks exactly like continuing to play, which is why giving in occasionally is worse than giving in every time.