Daniel is constructing a fence that consists of parallel sides ab and ef. complete the proof to explain how he can show that the measure of angle gkb = 120° by filling in the missing justifications.

option a: 1. alternate interior angles theorem; 2. definition of congruence

option b: 1. alternate exterior angles theorem; 2. substitution property

option c: 1. definition of congruence; 2. alternate interior angles theorem

option d: 1. substitution property; 2. alternate exterior angles theorem