S1: The machines that drive modern 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 forestes, water, wind and withered vegetables.
The proper sequence should be