ANS:C - both zero and hundred

The percentage humidity is less than the relative humidity only at zero percent humidity. Here's why:

• Relative humidity (RH) is the ratio of the partial pressure of water vapor in the air to the saturation vapor pressure at a given temperature. It is expressed as a percentage.
• Percentage humidity is not a standard term in meteorology or physics. However, it can be assumed to refer to the same concept as relative humidity.
• At zero percent humidity, there is no water vapor present in the air. Therefore, both the relative humidity and the percentage humidity are zero.
• At all other humidity levels, the relative humidity and percentage humidity will be equal, as they are both calculated using the same principle of comparing the actual water vapor content in the air to the maximum amount of water vapor the air can hold at a given temperature.