7-72 Carla decided to turn her attention to rectangles. Knowing that a rectangle is defined as a quadrilateral with four right angles, she drew the diagram below. After some exploration, she conjectured: If a quadrilateral is a rectangle, then it is also a parallelogram. Help her prove that her rectangle ABCD must be a parallelogram. That is, prove that the opposite sides must be parallel. Then add this theorem to your Theorem Toolkit