Rachel's Dog Loves Meat and Salivates When He Sees It

Rachel's dog loves meat and salivates when he sees it.Rachel repeatedly shows her dog a picture of a circle before showing him a piece of meat.Soon,the dog salivates to the picture of the circle.When Rachel shows her dog a picture of an oval,the dog does not salivate,which is best explained by

A) stimulus generalization.
B) stimulus discrimination.
C) spontaneous recovery.
D) second-order conditioning.