What is the minimum wind velocity required for obtaining useful energy from a windmill?

In a hydroelectric power plant,more electrical power can be generated if water falls from a greater height because:

Why are gaseous fuels preferred to solid fuels in chemical industries?

Write true or false for the following statement:
Glass cover in a solar heating device allows the short wavelength of ultraviolet radiations to enter the solar cooker.

Blowing wind carries kinetic energy. Mention the two factors that cause winds to blow.

On what basis would you classify energy sources as
$(a)$ renewable and non-renewable?
$(b)$ exhaustible and inexhaustible?
Are the options given in $(a)$ and $(b)$ the same?