Final answer:
The density of a gas increases when more gas is added to a constant-volume container, as the mass per unit volume increases. This effect is independent of the type of gas. The correct answer is a) Increases.
Step-by-step explanation:
When additional gas is added to a constant-volume container, the density of the gas increases. This is because density is defined as mass per unit volume, and by adding more gas (more mass) into the container without changing its volume, you increase the amount of mass in a given volume. According to Avogadro's law, at constant pressure and temperature, the volume would increase if more gas is added, but in this scenario, the volume is held constant, so the density must increase.
If we keep adding energy to this system, the gas particles will move faster, increasing the pressure inside the container as temperature rises. However, the density increase due to adding more gas is not dependent on the type of gas but the amount of gas added to the container.