Keaton wrote the following paragraph proof for the Vertical Angles Theorem: Line segment NT intersects line segment MR forming four angles. Angles 1 and 3 are vertical angles. Angles 2 and 4 are vertical

The sum of angle 1 and angle 4 and the sum of angle 3 and angle 4 are each equal to 180 degrees . The sum of angle 1 and angle 4 is equal to the sum of angle 3 and angle 4 by the transitive property of equality. Angle 1 is equal to angle 3 by the subtraction property of equality.

Which phrase completes the proof? (6 points)

by construction using a straightedge
by the definition of a perpendicular bisector
by the definition of supplementary angles.
by the vertical angles theorem