Why Does a Decrease in Demand Cause a Decrease in a Good's

Why does a decrease in demand cause a decrease in a good's equilibrium price?

A) There is a surplus at the old price, which leads firms to charge a lower price.
B) There is a shortage at the old price, which leads firms to charge a lower price.
C) A decrease in demand causes a price floor to be placed on a market.
D) At the old price, the quantity demanded is greater than the quantity supplied, which leads firms to charge a lower price.