Hardness of water is due to the presence of salts of

• A
  $Na^{+}$ and $K^{+}$
• B
  $Ca^{2+}$ and $Mg^{2+}$
• C
  $Ca^{2+}$ and $K^{+}$
• D
  $Ca^{2+}$ and $Na^{+}$