Answer the following question given this paragraph: 

The canister on the left contains a gas at a certain pressure. The attached air pump is then used to double the amount of gas in the canister. Since the canister cannot expand, the increased number of air molecules will strike the inside walls of the canister twice as frequently as they did before. The result is that the pressure inside the canister doubles. As you might imagine, if more and more air is continually added to a rigid container, it may eventually burst. Reducing the number of molecules in a rigid container has the opposite effect and the pressure decreases.


Q: Reducing the number of molecules in a rigid container causes what to decrease?

 Choices:

- velocity

- demand

- energy

- pressure

A:
pressure
