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: Newton was perhaps the greatest scientist that ever lived.
S6: Newton went home and worked quietly by himself for about 18 months.
P: But when he was only 22, a terrible plague epidemic swept over England.
Q: He was the son of a Lincolnshire farmer, and was born in 1642.
R: Therefore the universities were closed.
S: He went to Cambridge to study mathematics when he was 19.
The proper sequence should be: