The figure below shows a quadrilateral ABCD. Sides AB and DC are congruent and parallel: (image)

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

Angle DBC and angle BDA form a pair of vertical angles, not a pair of alternate interior angles, which are congruent.

Triangles ABD and CDB are congruent by the SAS postulate instead of the SSS postulate.

Triangles ABD and BCD are congruent by the AAS postulate instead of the SSS postulate.

Angle DBC and angle BDA form a pair of corresponding angles, not a pair of alternate interior angles, which are congruent.