We are asked to say how the contractor can prove that the two hand rails are parallel. First, he should check if both vertical supports are perpendicular to the ground. If they are, we know that they are parallel to each other.
After that, he should measure the distance between the two hand rails, both at the top of the steps and at the bottom of them. If they are equal, then we have one pair of congruent parallel sides of a quadrilateral formed using the vertical supports and hand rails.
From the Conditions of Parallelograms, we know that if one pair of opposite sides of a quadrilateral is both parallel and congruent, then the quadrilateral is a parallelogram. Therefore, the hand rails, as they are the second pair of opposite sides of this parallelogram, are congruent and parallel.
