Please explain your answers as well. Thanks.
1. Which statement is true?
a.every rhombus is a square.
b.every trapezoid is a parallelogram.
c.diagonals are perpendicular.
d.adjacent sides are congruent.

2.Which statement is always true about a parallelogram?
a.Diagonals bisect each other.
b. diagonals bisect the angles.
c.diagonals are perpendicular.
d.adjacent sides are congruent.
I know the answer is a.diadonals bisect each other, but what's the explanation?

3.In quadrilateral ABCD, if line AB=DC and AD=BC, then diagonals AC and BD must...
a.be parallel.
b.be perpendicular.
c.bisect each other.
d.be congruent.

4.Which statement is not true for any given parallelogram ABCD?
a.Line AC is perpendicular to DB
b.Angle A = angle C
c. AB=DC
d.measure of angle B+ measure of angles C=180

Explain, please.
Thanks.