ANS:A - beginning of falling

The "beginning of falling rate" refers to the initial stage of the drying process where the rate of moisture removal from the material starts to decrease. Here's an explanation:

1. Initial stage: At the start of the drying process, the material contains a significant amount of moisture, and the rate of moisture removal is relatively high. This stage is often referred to as the "preheating" stage, where the material absorbs heat from the surrounding environment, leading to an increase in temperature.
2. Beginning of falling rate: As the drying process continues, the surface moisture evaporates, creating a boundary layer of moist air around the material. Initially, the moisture removal rate is primarily governed by external factors such as heat transfer from the surroundings to the material surface. This results in a relatively rapid decrease in moisture content.
3. Transition to falling rate period: The "beginning of falling rate" marks the transition point where the rate of moisture removal starts to slow down. At this stage, the moisture content gradient within the material becomes more significant, leading to increased resistance to moisture diffusion from the interior to the surface. Additionally, as the surface moisture decreases, the driving force for further moisture removal diminishes, contributing to the decrease in the moisture removal rate.
4. Factors affecting the beginning of falling rate: The specific point at which the beginning of the falling rate occurs depends on various factors such as the material properties (e.g., porosity, permeability), drying conditions (e.g., temperature, airflow rate), and the moisture content gradient within the material.

In summary, the "beginning of falling rate" represents the initial phase of the drying process where the rate of moisture removal gradually decreases as the material's surface moisture is depleted, leading to a transition to the falling rate period of drying.