Which of the Following Statements Is an Example of a Reinforcement

Which of the following statements 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, you should knock before entering.
C) Hotdogs and peanuts go together, because both are sold at baseball games.
D) If the recipe says to preheat the oven, you should turn it on before mixing the ingredients.