Theorem: The segment joining the midpoints of two sides of a triangle is parallel to the third side and half its length. A two-column proof of the theorem is shown, but the proof is incomplete.

A triangle with vertices A is at 6, 8. B is at 2, 2. C is at 8, 4. Segment DE with point D on side AB and point E is on side BC

Statement Reason
The coordinates of point D are (4, 5) and coordinates of point E are (5, 3) By the midpoint formula
Length of segment DE is Square root of 5 and length of segment AC is 2 multiplied by the square root of 5
Segment DE is half the length of segment AC By substitution
Slope of segment DE is −2 and slope of segment AC is −2 By the slope formula
Segment DE is parallel to segment AC Slopes of parallel lines are equal
Which of the following completes the proof?

By the addition property
By the distance formula
By construction
Given