ANS:C - both (a) and (b)

For constant rate drying, the effects of velocity, temperature, and humidity of the gas, as well as the thickness of the solid, are the same. In constant rate drying, the rate of drying is primarily controlled by the rate at which moisture can be transferred from the interior of the solid to its surface. During this stage, the drying process is not limited by the rate of heat or mass transfer at the surface of the solid, but rather by the rate of moisture diffusion within the solid itself. During constant rate drying, the following conditions apply:

• The velocity, temperature, and humidity of the gas: These factors affect the rate of moisture diffusion within the solid, but they do not significantly influence the overall rate of drying during this stage.
• The thickness of the solid: Since the drying process is controlled by moisture diffusion within the solid, changes in the thickness of the solid will affect the distance that moisture needs to travel to reach the surface for evaporation. However, the overall mechanism of drying remains the same regardless of the thickness.