In this question, there is a sentence of which some parts have been jumbled up. You are required to rearrange these parts which are labelled P, Q, R and S to produce the correct sentence. Choose the proper sequence and mark in your Answer Sheet accordingly.

Researchers    found that allergic reactions are brought about (P)   in the bloodstream (Q)     in the 1940s (R)      by the liberation of small quantities of a substance called histamine (S)

The correct sequence should be

Options