In an experiment studying classical conditioning, a tone sounds and 5 seconds later a mild skin shock is delivered to a participant. the skin shock results in a stress response consisting of a temporary increase in heart rate. this process is repeated once every 5 minutes. after a number of trials, the participant demonstrates an elevated heart rate when the tone sounds and before the shock is actually delivered. within the context of this example, what is the UR?

a. the oscillating increases and decreases in heart rate

b. the increase in heart rate to either the shock or the tone

c. the increase in heart rate after the tone but before the shock

d. the increase in heart rate after the shock