The following six sentences come from a paragraph. The first and the last sentences are given. Choose the order in which the four sentences (PQRS) should appear to complete the paragraph.
S1: Before man settled down, his life was not governed by the seasons.
S6: However, we do not know when man first began to divide the day into twenty-four hours, and the hour into sixty minutes.
P: So man began to measure time, and first he looked for convenient units of measurement, one of which was the day.
Q: This was done by making a mark for each day on a tree trunk.
R: Once he settled down to the agricultural way of life, he had to be able to calculate the change of seasons on which agriculture depended.
S: In order to calculate the passing of time, it was necessary for man to record the number of days in each season.
The proper sequence should be: