The figure below shows a parallelogram ABCD. Side AB is parallel to side DC, and side AD is parallel to side BC

A student wrote the following sentences to prove that parallelogram ABCD has two pairs of opposite sides equal

For triangles ABD and CDB, alternate interior angle ABD is congruent to angle CDB because AB and DC are parallel lines. Similarly, alternate interior angle ADB is equal to angle CBD because AD and BC are parallel lines Dil is equal to DB by the reflexive property. Therefore, triangles ABD and CDB are congruent by the SAS postulate. Therefore AB is congruent to DC and AD is congruent to BC by CPCTC

Which statement best describes a flaw in the student's proof?

Answer is: option D
Triangles ABD and CDB are congruent by the ASA postulate instead of the SAS postulate
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