Why is $CNG$ considered an environmentally friendly fuel?

Choose the only renewable source of energy from the following:

$(a)$ Magnetic field lines of two bar magnets $A$ and $B$ are as shown below. Name the poles of the magnets facing each other.
$(b)$ Two magnetic field lines never intersect each other. Why?
$(c)$ How does the strength of the magnetic field at the centre of a current-carrying circular coil depend on the
$(i)$ radius of the coil,
$(ii)$ number of turns in the coil, and
$(iii)$ strength of the current flowing in the coil?

"$A$ biogas plant solves the fuel as well as environmental problems". Discuss.

Minimum velocity of wind required for a functional windmill is about:

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