I guess this kind of questions usually appear as the first questioin, and is better to use P.O.E (process of elimination) througth applying all conditions offered by the stimulus to get the best answer.
你的第三条和第五条矛盾呀,题对吗? N=R-->=R, N=R-->O=R, then how can P=R-->O=S when N=R?
if N at R, then P at R (i.e. NR-->R), which only means PR is necessary for NR but not sufficient for it. So if PR, N is not necessarily at R(i.e. PR-->NR is invalid), so PR-->NR-->OR is invalid.