A gas with a vapor density of __ will rise in air.

A gas with a vapor density less than 1.0 will rise in air because it is lighter than the surrounding air. Vapor density is a comparison of the density of a gas to that of air, which has a vapor density of approximately 1.0.

When a gas has a vapor density of 0.5, for example, it is less dense than the air and will tend to rise. Conversely, if a gas has a vapor density greater than 1.0, such as 1.5, it is denser than air and will tend to sink. Therefore, a vapor density of 0.1 indicates that the gas is significantly lighter than air, making it very buoyant. This is what allows it to rise in the atmosphere.

In summary, the correct choice is based on the understanding that gases with vapor densities below 1.0, such as 0.1, will rise, while those at or above this threshold will not exhibit that same behavior.