ANS:B - increasing temperatures both on the surface and within the solid.

The falling rate period in the drying of a solid is characterized by: None of these. During the falling rate period:

1. Rate of Drying: The rate of drying decreases over time as the moisture content of the solid decreases. This is because as the surface of the solid dries, the moisture must travel from the interior to the surface through capillaries or pores, which slows down the drying process.
2. Temperature Changes: The temperature on the surface and within the solid may not necessarily increase during the falling rate period. In fact, the temperature of the solid may remain relatively constant or decrease slightly as the moisture evaporates and carries away heat from the solid. Additionally, if the drying process is conducted under constant conditions, such as constant temperature and humidity, there may not be significant temperature changes.

During the falling rate period, the drying process is primarily controlled by the rate of moisture diffusion within the solid, rather than changes in temperature. Therefore, the characteristic feature of the falling rate period is the decreasing rate of drying as the moisture content decreases, rather than temperature changes.