To confirm that a quadrilateral with vertices P, Q, R and S is a parallelogram, which single calculation would be the MOST appropriate?
(a) Show that the diagonals PR and QS are equal in length.
(b) Show that one pair of opposite sides, say PQ and RS, have the same length.
(c) Show that the midpoints of PR and QS coincide.
(d) Show that the slope of PQ equals the slope of RS.