Sensory Preconditioning Challenges the Criterion of Differential Biological Strength in Associative

Sensory preconditioning challenges the criterion of differential biological strength in associative learning due to which of the following?

A) Only the US has much biological strength in sensory preconditioning procedures.
B) Only the CS has much biological strength in sensory preconditioning procedures.
C) Neither of the stimuli being associated has much biological strength in sensory preconditioning procedures.
D) Both of the stimuli being associated have biological strength in sensory preconditioning procedures.