The number of diagonals in a polygon of m sides is

• A

  $\frac{1}{2!}\mathrm{m}(\mathrm{m}-5)$

• B

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

• C

  $\frac{1}{2!}\mathrm{m}(\mathrm{m}-3)$

• D

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