Geothermal energy is produced by the:
(a) fission of radioactive materials
(b) burning of coal inside the coal mines
(c) combustion of natural gas deep inside the earth
(d) fusion of radioactive substances
A certain form of energy which is not sourced directly or indirectly from the sun and does not cause any pollution is very easily converted into electricity. This form of energy is, however, not available everywhere. Moreover, it is technically very difficult and expensive to obtain it. Name the form of energy.