What Is the Major Difference Between a Conditioned Stimulus and an Unconditioned

What is the major difference between a conditioned stimulus and an unconditioned stimulus

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