When Calculating the Price Elasticity of Demand,which of the Following

When calculating the price elasticity of demand,which of the following conditions must be satisfied?

A) All other factors that influence demand must be held constant.
B) Prices of related goods must be held constant but all other factors must be allowed to vary.
C) Prices of related goods must be allowed to vary but all other factors must be held constant.
D) All other factors than influence demand must be allowed to vary.