The following is an incomplete paragraph proving that the opposite sides of parallelogram ABCD are congruent: Parallelogram ABCD is shown where segment AB is parallel to segment DC and segment BC is parallel to segment AD.

According to the given information, segment AB is parallel to segment DC and segment BC is parallel to segment AD. Construct diagonal A C with a straightedge. It is congruent to itself by the Reflexive Property of Equality. . Angles BCA and DAC are congruent by the same reasoning. Triangles BCA and DAC are congruent according to the Angle-Side-Angle (ASA) Theorem. By CPCTC, opposite sides AB and CD, as well as sides BC and DA, are congruent.

Which sentence accurately completes the proof? (5 points)

a
Angles ABC and CDA are corresponding parts of congruent triangles, which are congruent (CPCTC).

b
Angles ABC and CDA are congruent according to a property of parallelograms (opposite angles congruent).

c
Angles BAC and DCA are congruent by the Same-Side Interior Angles Theorem.

d
Angles BAC and DCA are congruent by the Alternate Interior Angles Theorem.