In this section, each item consists of six sentences of a passage. The first and sixth sentences are given as SI and S6. The middle four sentences in each have been jumbled up and labeled as P, Q, R, and S. You are required to find the proper sequence of the four sentences and mark your response accordingly.

S1: The metals in the middle of the activity series such as iron, zinc, lead, and copper, are moderately reactive.
P:  The sulphide ores are converted into oxides by heating strongly in the presence of excess air. 
Q: Therefore, prior to reduction, the metal sulphides and carbonates must be converted into metal oxides.
R: It is easier to obtain metal from its oxide, as compared to its sulphides and carbonates.
S: These are usually present as sulphides or carbonates in nature.
S6: This process is known as roasting.