In the following question, there are six sentences marked S1, S6, P, Q, R, S. The positions of S1 and S6 are fixed. You are required to choose one of the four alternatives which would be the most logical sequence of the sentence in the passage.

S1. The mail is first collected from different letter boxes.

P: From there it is sent to the head post office.

Q: It is then sorted out at the sorting office.

R: The mail is again sorted out at the head office by the concerned beat postman.

S: The sorted mail is sent to the zonal post office.

S6. Finally it is delivered to us.