Prove: The quadrilateral formed by joining in order the midpoints of the sides of a rectangle is a parallelogram. Given: ABCD is rectangle K, L, M, N are midpoints Prove: KLMN is a parallelogram Which of the following reasons would complete the proof in line 6? If both pair of opposite sides of a quadrilateral are equal, then it is a parallelogram. If both pair of opposite sides of a quadrilateral are parallel, then it is a parallelogram. If a pair of opposite sides of a quadrilateral are parallel and equal, then it is a parallelogram.