After a Single Bad Encounter with a Dog,a Child Develops

After a single bad encounter with a dog,a child develops a dog phobia.She shows strong avoidance and escape behaviours at the sight of a dog.Initially,the sight of a dog served as a conditioned stimulus that triggered fear.However,after operant conditioning has taken over and her avoidance behaviour has been learned,the sight of a dog would best be considered as an example of a(n) :

A) conditioned response.
B) negative reinforcer.
C) unconditioned stimulus.
D) antecedent stimulus.