A flow chart has 4 boxes. The top box is labeled given and contains angle A is-congruent-to angle B, angle C is congruent to angle B. The second box is labeled symmetric property and contains angle B is-congruent-to angle C. The third box is labeled question mark and contains angle A is congruent to angle C. The fourth box is labeled definition of congruent to angle x and contains m angle A = m angle C. What is the missing justification?