The charge/size ratio of a cation determines its polarizing power. Which of the following sequences represents the increasing order of the polarizing power of the cations $K^+$,$Ca^{2+}$,$Mg^{2+}$,and $Be^{2+}$?

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