Bryson Was Three When His Neighbor's House Burned Down

Bryson was three when his neighbor's house burned down.He heard the loud sirens and saw the fire and trembled with fear.Now he's starting preschool.He hears the loud bell ring and associates it with the fire and fire trucks and begins to tremble.In this setting,the sound of the bell is:

A) a neutral response.
B) a neutral stimulus.
C) an unconditioned response.
D) a conditioned stimulus.