This question consists of a sentence, parts of which have been jumbled. These parts have been labelled as P, Q, R and S. Given below each sentence are four sequences, namely (a), (b), (c) and (d). You are required to re-arrange the jumbled parts of the sentence and mark your response accordingly. 

in assisting astronomers (P)/ constellations are useful (Q)/ and navigators (R)/ to locate certain stars (S)/