Given below are six sentences, four of which (P,Q,R,S) are jumbled. Pick the option that gives the correct order of P,Q,R,S.

S-1 If you are doing a piece of writing, you need to refer to some books for information on the subject.

P - For this, you need to know how the books are classified and arranged in the library.

Q- This necessitates your using a library and locating the book you need.

R - The catalogue in the library is most often available in form of cards

S - This becomes easy and saves time if you understand how the catalouging system in the library 

S-6 The cards are arranged in alphabetical order.