In an Experiment, a Pigeon Receives Food Whenever It Pecks

Multiple Choice

In an experiment, a pigeon receives food whenever it pecks a green key and not when it pecks a yellow key. In this experiment, the green key acts as the _____ while the yellow key acts as the _____.

A) S^D; S^∆
B) S^∆; S^D
C) S^D; S^R
D) S^R; S^D