A parallelogram is a quadrilateral with two pairs of parallel sides. A quadrilateral is a parallelogram _[blank]_.

Which answer correctly fills in the blank in the previous sentence?

A) because it has only four sides and two of them are parallel
B) if it has two pairs of parallel sides and is a polygon
C) if it has only two pairs of parallel sides
D) if and only if it has two pairs of parallel sides