Below is a two-column proof incorrectly proving that the three angles of δpqr sum to 180°:
draw line zy parallel to segment pq
m∠zrp + m∠prq + m∠qry = m∠zry
∠zrp ≅ ∠rpq
∠qry ≅ ∠pqr
m∠rpq + m∠prq + m∠pqr = m∠zry
m∠zry = 180°
m∠rpq + m∠prq + m∠pqr = 180°
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 before the construction of line zy
c. the three angles of δpqr equal 180° according to substitution
d. line zy should be drawn parallel to segment qr