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

Let the angle in the condition be A. Its two consecutive angles, B and D, are each supplementary to A, so B = D. Since the angles of a quadrilateral total 360°, the remaining angle C equals A. Thus both pairs of opposite angles are congruent, which proves the quadrilateral is a parallelogram. The correct choice is B: one angle is supplementary to both consecutive angles.

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