Given that a shape is a parallelogram, which of the following is sufficient to state that it is also a rectangle?

In a parallelogram, the diagonals already bisect each other, and opposite sides are equal and parallel, so choices A, C, and D do not establish that it is a rectangle. A parallelogram with diagonals of equal length is a rectangle. Therefore, the sufficient condition is that the diagonals are of equal length.

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