Two different colours are exhibited by the complex, $\left[\mathrm{Co}{\left({\mathrm{NH}}_3\right)}_4{\mathrm{Cl}}_2\right]\mathrm{Cl}$ which is due to

• A

  Existence of two ionization isomers

• B

  Existence of two linkage isomers

• C

  Existence of two optical isomers

• D

  Existence of two geometrical isomers