Below is a two-column proof incorrectly proving that the three angles of δpqr sum to 180°:

statements reasons

∠qry ≅ ∠pqralternate interior angles theorem
draw line zy parallel to segment pq construction
m∠zrp + m∠prq + m∠qry = m∠zry angle addition postulate
∠zrp ≅ ∠rpq alternate interior angles theorem
m∠rpq + m∠prq + m∠pqr = m∠zry substitution
m∠zry = 180° definition of a straight angle
m∠rpq + m∠prq + m∠pqr = 180° substitution

which statement will accurately correct the two-column proof?

a.) the measure of angle zry equals 180° by definition of supplementary angles.
b.) angles qry and pqr should be proven congruent after the construction of line zy.
c.) the three angles of δpqr equal 180° according to the transitive property of equality.
d.) line zy should be drawn parallel to segment qr.