Choose from the options given below:

S1 introduces the subject of the paragraph, the different ways in which volcanoes are classified. P follows S1 as it talks about most commonly used way of classification. R follows P as �This classification� in R refers to �the most common and certainly the one used by non-specialists� in P. Q follows R as it defines when a volcano is �typically� said to be extinct after mentioning that the �typical� definition is problematical.Thus, the correct sequence is PRQ.