Given: ABCD are vertices of quadrilateral ABCD.
Prove: ABCD is a trapezoid.
Statements:

ABCD are vertices of quadrilateral ABCD.
Slope of AB: m₁, Slope of BC: m₂, Slope of CD: m₃, Slope of DA: m₄.
?
ABCD is a trapezoid.
Reasons:
Given
Definition of slope
?
Definition of a trapezoid
What is step 3 to complete the proof?
a. By the definition of parallel lines
b. By the definition of perpendicular lines
c. By the definition of perpendicular lines
d. By the definition of parallel lines