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

Match List $I$ with List $II$ and select the correct answer using the codes given below the lists:

$1.$ Heavy water	$a.$ Bicarbonates of $Mg$ and $Ca$ in water
$2.$ Temporary hard water	$b.$ No foreign ions in water
$3.$ Soft water	$c.$ $D_2O$
$4.$ Permanent hard water	$d.$ Sulphates and chlorides of $Mg$ and $Ca$ in water

Match the following:

List-$I$ (Reactions)	List-$II$ (Methods)
$A$. $Mg(HCO_3)_2 \rightarrow Mg(OH)_2 \downarrow + 2 CO_2 \uparrow$	$I$. Clark's method
$B$. $M^{2+} + Na_4P_6O_{18}^{2-} \rightarrow [Na_2MP_6O_{18}]^{2-} + 2 Na^{+}$	$II$. Ion exchange method
$C$. $Ca(HCO_3)_2 + Ca(OH)_2 \rightarrow 2 CaCO_3 + 2 H_2O$	$III$. Boiling
$D$. $2 NaZ + Ca^{2+}_{(aq)} \rightarrow 2 Na^{+} + CaZ_2$ ($Z$ = Zeolite)	$IV$. Calgon's method

Given below are two statements:
Statement-$I$: Permutit process is more efficient compared to the synthetic resin method for the softening of water.
Statement-$II$: Synthetic resin method results in the formation of soluble sodium salts.
In the light of the above statements,choose the most appropriate answer from the options given below:

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

What is the importance of heavy water?