State whether the following statement is true or false:
Spherical reflectors are used to obtain high temperatures in solar heating devices.

Ranvir lives in a village and uses wood as a fuel. He studied in his school that charcoal is a better fuel than wood. He decided to use charcoal instead of wood.
$(a)$ How can Ranvir obtain charcoal from wood?
$(b)$ Why is charcoal considered a better fuel than wood? (Give two reasons)

Name the three forms in which energy from oceans is made available for use. What are $OTEC$ power plants? How do they operate?

In a box-type solar cooker,what is the range of temperature that can be achieved in two or three hours?

Mention two disadvantages of using animal dung directly as a fuel.

Why are biogas plants considered to be a boon to farmers? Give reasons.