ANS:A - perfectly dry

To remove all the moisture from a wet solid, it requires exposure to perfectly dry air. Explanation:

• When drying a wet solid, the goal is to remove moisture from the solid until it reaches a desired level of dryness.
• Perfectly dry air refers to air that contains no moisture or humidity. It has a relative humidity of 0%.
• By exposing the wet solid to perfectly dry air, a moisture gradient is established between the solid surface and the surrounding air.
• According to the principles of mass transfer, moisture will naturally diffuse from regions of higher concentration (the wet solid) to regions of lower concentration (the dry air) until equilibrium is reached.
• Therefore, exposing the wet solid to perfectly dry air ensures the most efficient and thorough removal of moisture from the solid, as there is a strong driving force for moisture to evaporate into the dry air.
• Exposure to highly humid air or high temperature alone may not be sufficient to remove all moisture from the wet solid, as there may still be a significant moisture gradient between the solid and the air, limiting the drying process.