Russell's Mom Wants Him to Study for Two Hours Each

Multiple Choice

Russell's mom wants him to study for two hours each night,so she gives him treats during the evening.Sometimes she gives him the treats after ten minutes,sometimes after thirty minutes.On the average,Russell is given a treat every twenty minutes.Which reinforcement schedule is Russell's mom utilizing?

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