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. In the eighteenth century people expected most of their children to die before they were grown up.

P: Improvement began at the beginning of the nineteenth century, chiefly owing to vaccination.

Q: The general death rate in 1948 (10.8) was the lowest ever recorded till that date.

R: In 1920 the infant mortality in England and Wales was 80 per thousand, in 1948 it was 34 per thousand.

S: It has continued ever since and is still continuing.

S6. There is no obvious limit to the improvement of health that can be brought about by medicine.