6.

Quadrilateral ABCD is a parallelogram. If adjacent angles are congruent, which statement must be true?


a. 

Quadrilateral ABCD is a square.


b. 

Quadrilateral ABCD is a rhombus.


c. 

Quadrilateral ABCD is a rectangle.


d. 

Quadrilateral ABCD is an isosceles trapezoid.
