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

During the constant rate period of drying, the rate of drying is primarily controlled by the rate of moisture transfer from the interior of the material to its surface. This period occurs when the moisture content in the material is relatively high, and the surface is not yet saturated with moisture. Now, let's analyze the options: (a) Decrease in air temperature: A decrease in air temperature generally decreases the vapor pressure gradient between the moist material and the surrounding air. This reduction in the vapor pressure gradient can slow down the rate of moisture transfer from the material to the air, thus decreasing the rate of drying. So, this statement is correct. (b) Increase in air humidity: An increase in air humidity means that the air already contains a higher amount of moisture. This reduces the vapor pressure gradient between the material and the air, which in turn reduces the driving force for moisture transfer. Consequently, an increase in air humidity tends to decrease the rate of drying. So, this statement is also correct. Therefore, the correct answer is: (c) Both (a) and (b) - The rate of drying decreases with both a decrease in air temperature and an increase in air humidity during the constant rate period.