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Given a scientific passage and an answer, generate a question for the given answer.

Passage: Recall from the kinetic-molecular theory that gas particles move randomly and in straight lines until they elastically collide with either other gas particles or with one of the walls of the container. It is these collisions with the walls of the container that defines the pressure of the gas. Four variables are used to describe the condition of a gas. They are pressure , volume , temperature , and the amount of the gas as measured by the number moles . We will examine separately how the volume, temperature, and amount of gas each affect the pressure of an enclosed gas sample. Answer: four
Output:
How many variables are used to describe the condition of a gas?