ANS:C - does not change with time,

During the constant rate drying period, the vaporization rate per unit drying surface area typically decreases with time. This phenomenon occurs because, during the constant rate drying period, the rate of moisture evaporation from the wet solid is primarily governed by the diffusion of moisture from the interior of the solid to the surface. As the drying process progresses, the moisture content gradient within the solid decreases, leading to a reduced driving force for moisture diffusion. Consequently, the rate at which moisture reaches the surface and evaporates into the surrounding air diminishes over time. Therefore, the correct completion to the statement is: During the constant rate drying period, vaporization rate per unit drying surface area decreases with time.