The figure below shows a quadrilateral abcd. sides ab and dc are equal and parallel: a student wrote the following sentences to prove that quadrilateral abcd is a parallelogram: side ab is parallel to side dc so the alternate interior angles, angle abd and angle cdb, are congruent. side ab is equal to side dc and db is the side common to triangles abd and bcd. therefore, the triangles abd and bcd are congruent by sas postulate. by cpctc angles dbc and adb are congruent and sides ad and therefore, ad is parallel and equal to bc. quadrilateral abcd is a parallelogram because its bc are congruent. angle dbc and angle adb opposite sides are equal and parallel which phrase best completes the student's proof? are congruent by the aas postulate are congruent by the asa postulate form a pair of alternate interior angles which are congruent form a pair of vertical angles which are congruent