Best answer gets

the figure below shows a quadrilateral abcd. sides ab and dc are equal and parallel:

a quadrilateral abcd is shown with the opposite sides ab and dc shown parallel and equal.

a student wrote the following sentences to prove that quadrilateral abcd is a parallelogram

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

triangles abd and bcd are congruent by the sss postulate instead of the sas postulate.
triangles abd and bcd are congruent by the aas postulate instead of the sas postulate.
angle dbc and angle bda form a pair of corresponding angles, not a pair of vertical angles.
angle dbc and angle bda form a pair of alternate interior angles that are congruent, not a pair of vertical a