If m parallel lines in a plane are intersected by a family of n parallel lines, the number of parallelograms that can be formed is

• A

  $\frac{1}{4}\mathrm{mn}(\mathrm{m}-1)(\mathrm{n}-1)$

• B

  $\frac{1}{2}\mathrm{mn}(\mathrm{m}-1)(\mathrm{n}-1)$

• C

  $\frac{1}{4}{\mathrm{m}}^2{\mathrm{n}}^2$

• D

  none of these