The first and the sixth sentence of the passage are given in the beginning as SI and S6. The middle four sentences have been removed and jumbled up. These are labelled P, Q, R and S. Choose the correct order of the sentences.
SI : The machines that drive modem civilisation derive their power from coal and oil.
S6: Nuclear energy may also be effectively used in this respect.
P : But they are not inexhaustible.
Q : These sources may not be exhausted very soon.
R: A time may come when some other sources have to be tapped and utilised.
S: Power may, of course, be obtained in future from forests, water, wind and withered vegetables.
The proper sequence should be