Julio served in the war in Iraq and was severely traumatized when a rocket-propelled grenade exploded next to his Humvee. Recently, Julio was studying in the library and a large book fell off a high shelf and banged on the floor. The noise brought Julio instantly to his feet. In terms of classical conditioning, his response can best be explained by:.
a. spontaneous recovery.
b. negative reflex.
c. stimulus generalization.
d. reinforcement.