Which of the following conditions in a quadrilateral proves it is a parallelogram?

a.) opposite sides are parallel
b.) opposite sides are equal in length
c.) opposite angles are equal (angles "a" are the same, and angles "b" are the same)
d.) angles "a" and "b" add up to 180°, so they are supplementary angles.

For this item, only the fourth choice do not prove that the given figure is a parallelogram. For a quadrilateral to be parallelogram, its opposite sides should be equal and are equal in length. The opposite angles should also be equal; however, they are not always supplementary. 

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