The correct answer is A: A trapezoid has only one pair of parallel sides, but a parallelogram has two.
To elaborate:
- A parallelogram is defined as a quadrilateral with two pairs of parallel sides.
- A trapezoid (in some definitions, particularly in the U.S.) is defined as a quadrilateral with at least one pair of parallel sides, which means it has only one pair of parallel sides.
The other options are incorrect in their statements about the properties of these shapes.