In the opening sentence, the discovery of 'fire' has been discussed which will be followed by Q as it relates past to the present with the word 'now'. S shows a contradiction and will come after Q. P and R form a mandatory pair taking the context to the past. Also, R will clearly precede the closing line to complete the passage aptly. The correct sequence of the passage so formed will be QSPR. So, option (b) is correct.