Which Statement Is an Example of a Reinforcement Contingency

Multiple Choice

Which statement is an example of a reinforcement contingency?

A) A child's mother will give him what he wants,but only if he says "please."
B) When a door is closed,one must should knock before entering.
C) Hot dogs and peanuts are related,because both are sold at baseball games.
D) A piece of food is delivered to a rat 2 seconds after a tone is presented.