Which process is used for the removal of hardness of water?

Statements about heavy water are given below.
$A.$ Heavy water is used in exchange reactions for the study of reaction mechanisms.
$B.$ Heavy water is prepared by exhaustive electrolysis of water.
$C.$ Heavy water has higher boiling point than ordinary water.
$D.$ Viscosity of $H_2O$ is greater than $D_2O$.

Boiling of hard water is helpful in removing the temporary hardness by converting calcium hydrogen carbonate and magnesium hydrogen carbonate to ....

The solubility of an ionic compound is compared in heavy water $(D_2O)$ and simple water $(H_2O)$. It is:

What causes the temporary and permanent hardness of water?

Which can be used to prepare pure and demineralised $H_2O$ ?