Petra and Huy are doing a class demonstration to show that solids, liquids, and gases all have mass. They use a balance for the solids and the liquids, but they are having trouble measuring the mass of gasses. Which is the best way for them to show that air has mass? A. Compare the masses of an air-filled balloon and an empty balloon. B. Use a fan to make a current of air and measure it. C. Put an empty bottle on one side and a full bottle on the other. D. Heat water to show it changing from liquid to water vapor.