In general, which is denser?

The concept of density in air relates to its composition and temperature. Dry air is generally denser than humid air because water vapor, which is lighter than the nitrogen and oxygen that make up most of dry air, displaces these heavier molecules when moisture is present. As humidity increases, the proportion of lighter water vapor increases, leading to a decrease in the overall density of the air mixture.

When considering temperature, cold air is denser than warm air, as the molecules in cold air are more closely packed together. However, in this context, the key comparison is between humid and dry air. Thus, dry air, being less influenced by the presence of lighter water vapor, is more dense than humid air.

This understanding helps clarify why the correct choice is dry air in the context of density comparisons. While cold air can also be denser than warm air, it is not the primary focus of the question comparing the density of humid and dry air.