Charles' law states that the volume of an enclosed gas is directly proportional to its temperature on which scale?

1. Charles' law holds when the pressure and the amount of gas are both kept constant. 2. Volume is directly proportional to the Kelvin temperature of the gas. 3. On the Celsius scale the zero of temperature does not line up with the zero of volume. 4. That is why temperatures must be converted to kelvin before this law is used. _Source: Siyavula Physical Sciences Grade 11 (Everything Science, CC BY 4.0), Chapter 7: Ideal gases_