What Is the Single Best Way to Ensure a Strong

What is the single best way to ensure a strong conditioned response,according to Rescorla

A)  Ensure that the conditional stimulus remains constantly present throughout training. 
B)  Present the conditioned stimulus sometimes when the unconditioned stimulus is not present. 
C)  Ensure that the conditioned stimulus is the most predictive signal for the unconditioned stimulus. 
D)  Ensure that the conditional stimulus is well above the absolute threshold of the animal being trained.