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

Question

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