question_answer

There is a set of m parallel lines intersecting a set of another n parallel lines in a plane. The number of parallelograms formed, is

A)  \[^{m-1}{{C}_{2}}.{}^{n-1}{{C}_{2}}\]  

B)  \[^{m}{{C}_{2}}.{}^{n}{{C}_{2}}\]

C)  \[^{m-1}{{C}_{2}}.{}^{n}{{C}_{2}}\]             

D)  \[^{m}{{C}_{2}}.{}^{n-1}{{C}_{2}}\]