Decide if the following statements are true or false. If they are true, explain how you know. If they are false, provide a counterexample.

1. If a quadrilateral has two sides that are parallel and two sides that are congruent, then the quadrilateral must be a parallelogram.

   Answer (a):

   False (isosceles trapezoid)

2. If the interior angles of a polygon add up to $360^\circ$, then the polygon must be a quadrilateral.

3. If a quadrilateral has three right angles, then the quadrilateral must be a rectangle.

4. If the diagonals of a quadrilateral bisect each other, then the quadrilateral must be a rhombus.