Answer:
The answer is: "It has to be a rectangle. I measured the diagonals to be sure they were the same length."
Explanation:
The other properties are true of a rectangle as well, but they could also be other shapes.
"It has to be a rectangle. The opposite sides are the same length."
This could also be describing a rhombus or square.
"It has to be a rectangle because it is not a square."
This could also be a rhombus, trapezoid, or even kite.
"It has to be a rectangle. I made sure that the two 8-foot boards are parallel and that the two 6-foot boards are parallel."
This can describe any parallelogram. Just because the sides are congruent and parallel does not mean that there are four 90degree angles formed.
"It has to be a rectangle. I measured the diagonals to be sure they were the same length."
This is a property of rectangle diagonals. They will always be congruent.