ANS:D - both 'b' & 'c'.

During sensible heating of humid air:

• Relative humidity decreases because the air's capacity to hold moisture increases with temperature, so the same amount of moisture in the air becomes a smaller percentage of the air's total capacity.
• Dew point remains constant because it is a measure of the temperature at which air becomes saturated with moisture, regardless of the air's temperature.
• Dry bulb temperature increases as sensible heating adds heat to the air.
• Wet bulb temperature may increase or decrease depending on the initial conditions and the specific heating process. If the air is initially unsaturated (below 100% relative humidity), the wet bulb temperature will increase along with the dry bulb temperature. However, if the air is initially saturated (at 100% relative humidity), the wet bulb temperature will remain constant during sensible heating.