The following question consists of six sentences of a passage. The first and the sixth sentences are given in the beginning as S1 and S6. The middle four sentences in each have been jumbled up and labelled P, Q, R and S. You are required to find the proper sequence of the four sentences and mark your response accordingly.

S1: Civilization dawned when early man learnt how to produce heat and energy by burning wood.

S6: When they have been used, they cannot be replaced.

P: Then steam was used to produce electricity.

Q: In this century great use has been made of oil and natural gas and the use of atomic reactors has also provided another source of energy.

R: Much later, the first industrial revolution was based on the production of steam by burning coal.

S: But none of these fuels is renewable.

The correct sequence should be