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 labelled as P, Q, R and S. You are required to find the proper sequence of the four sentences and mark your response accordingly.

S1: Anodising is a process of forming a thick oxide layer of aluminium.
P: This aluminium oxide coat makes it resistant to further corrosion. 
Q: The resistance can be improved further by making the oxide layer thicker. 
R: During anodising, a clean aluminium article is made the anode and is electrolysed with dilute sulphuric acid.
S: Aluminium develops a thin oxide layer when exposed to air. 
S6: The oxygen gas evolved at the anode reacts with aluminium to make a thicker protective oxide layer.