Question 571359
"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 
I have eliminated "B" and "D", but I am unsure as to whether ABCD is square or rectangle. Could you kindly explain as to why it is either one? 
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A parallelogram's sides can be of unequal lengths, so it is not necessarily a square.  That leaves rectangle.