In a Study Designed to Improve Immune System Functioning of Rats

In a study designed to improve immune system functioning of rats, a researcher pairs a drug that strengthens the immune system with sweet water. After several trials, the drug is removed, but when the rats consume only the sweetened water, their immune system functioning improves. This example demonstrates a potentially important application of:

A) the principles of classical conditioning.
B) evidence of stimulus generalization.
C) the use of negative reinforcement.
D) the principles of operant conditioning.