Which of the following are necessary when proving that the opposite sides of a parallelogram are congruent? a. alternate interior angles are supplementary. b. opposite sides are parallel. c. opposite sides are perpendicular. d. alternate interior angles are congruent

First we have to understand what parallogram is .

A parallelogram is that quadrilateral, whose opposite sides are parallel and equal .

And alternate interior angles are congruent , not supplementary in parallelogram.

And neither the opposite sides are perpendicular .

So the correct option is B .

other answer is correct. i had used the radius of the radius somehow. a website: