General Studies · Floor or Box Puzzle

Seven boxes M, N, O, P, Q, R, and S are kept one over the other but not necessarily in the same order. Only three boxes are kept above S. Only one box is kept between R and S. Only three boxes are kept between R and Q. Q is kept at some place above S. N is kept immediately below Q. M is kept at one of the places above O. P is not kept immediately above or below R. Which box is kept at the lowermost position?