In triangle MNO, angle NMO and angle NOM are congruent. To prove segment MN and segment ON are congruent using a proof by contradiction, what is the assumption statement that starts the proof?

Segment MN is not congruent to segment ON.
Segment MN is congruent to segment ON.
Angle NMO and angle NOM are not congruent.
Angle NMO and angle NOM are congruent.