Relative to the Conditioned Stimulus (CS), When Should the Unconditioned

Multiple Choice

Relative to the conditioned stimulus (CS) , when should the unconditioned stimulus (US) appear for classical conditioning to occur?

A) simultaneously with the conditioned stimulus (CS)
B) immediately before the conditioned stimulus (CS)
C) immediately after the conditioned stimulus (CS)
D) instead of the conditioned stimulus (CS)