The figure below shows a parallelogram ABCD Side AB 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 and COB alternate interior angle ABD is congruent to angle CDB because AB and DC are parallel lines Similarly interior angle equal to angle CBD because AD and are parallel lines equal to DB by the reflexive property Therefore , triangles ABD and COB are congruent by the SAS postulate Therefore AB congruent to and AD congruent BC CPCTC Which statement best describes a flaw in the student's proof ?