Answer:
C - Opposite sides are congruent
D - Corresponding parts of congruent triangles are congruent
Explanation:
A parallelogram is a quadrilateral whose both pairs of opposite sides are parallel. In a parallelogram, the 2 sets of opposite sides are congruent. Also it has 2 sets of opposite angles congruent.
Therefore, the following are necessary when proving the opposite sides of a parallelogram are congruent.
C - Opposite sides are congruent
D - Corresponding parts of congruent triangles are congruent