Which salt does not cause hardness in 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 $D_2O$.

What is meant by 'demineralised' water and how can it be obtained?

What is the boiling point of heavy water (in $^{\circ} C$)?

Assertion $(A)$ : $NaCl$ is less soluble in heavy water than in ordinary water.
Reason $(R)$ : Dielectric constant of ordinary water is more than that of heavy water.
The correct answer is

Is demineralised or distilled water useful for drinking purposes? If not,how can it be made useful?