C. A substance is denser as a solid than as a gas.
For most substances, solids tend to have a higher density than liquids, and liquids generally have a higher density than gases. This is because the particles in solids are closely packed together, while in liquids they are less tightly packed, and in gases, they are far apart. However, exceptions exist, such as water, which is less dense as a solid (ice) than as a liquid.