Answer:
C. US and the CS must be presented very close together in time.
Explanation:
Classical conditioning: In psychology, the classical conditioning experiment was done by one the famous and influential psychologist named Ivan Pavlov.
According to Ivan Pavlov, the classical conditioning is defined as a form of the learning process in which a conditioned stimulus gets associated or connected with an unrelated or unconnected unconditioned stimulus to develop or create a behavioral response referred to conditioned response (formerly neutral stimulus which is being learned).
