The species $Ar$,$K^{+}$ and $Ca^{2+}$ contain the same number of electrons. In which order do their radii increase?

• A
  $Ca^{2+} < K^{+} < Ar$
• B
  $K^{+} < Ar < Ca^{2+}$
• C
  $Ar < K^{+} < Ca^{2+}$
• D
  $Ca^{2+} < Ar < K^{+}$