‘S’ will be the first part of the given sequence as it is talkingabout the form of treatment referred in the opening part of thegiven passage. ‘Q’ will come next as ‘this’ is used in the context of the ‘heating’ mentioned in ‘S’. It will be followed by ‘R’ as it tells the after effect of boiling that is, cooling of first water vapours which in turn provides ‘finest petrol’. ‘P’ will follow next as it talks about the fact that the gas that comes gets converted into paraffin. Hence, SQRP is the correct sequence. Thus, option (a) is correct.