Lecture 13 (Geometry) Similar Triangles and the Pythagorean Theorem

Which result is proved as a quick standalone theorem because the textbook simply assumes it?

1. A.The opposite sides of a parallelogram are congruent to each other.
2. B.The diagonals of a parallelogram bisect each other.
3. C.The angle sum of a parallelogram is 360 degrees.
4. D.Opposite angles of a parallelogram are supplementary.