The Major Difference Between a Conditioned Stimulus and an Unconditioned

The major difference between a conditioned stimulus and an unconditioned stimulus is

A) the order in which they are presented.
B) that one reliably elicits the response of interest prior to conditioning while the other does not.
C) that during conditioning the response to one increases while the response to the other decreases.
D) the strength of the response that each stimulus elicits.