Reasoning through steps of a proof
given: m || cb
prove: m∠abc + m∠bac + m∠acb = 180°

which statement justifies that angle xab is congruent to angle abc?

a) corresponding angles of parallel lines cut by a transversal are congruent.
b) vertical angles are congruent.
c) same-side interior angles of parallel lines cut by a transversal are supplementary.
d) alternate interior angles of parallel lines cut by a transversal are congruent.