The diagonals AC and BD of quadrilateral ABCD intersect at O. It is found that △AOB≅△COD by the SAS criterion. Which statement is certainly true?
a) ABCD is a rectangle.
b) ABCD has equal diagonals but need not be a parallelogram.
c) ABCD is a parallelogram.
d) No special conclusion can be drawn about ABCD.