Which of the following will cause softening of hard water?

Oxygen is slightly denser than air,so it is collected over:

Match the following methods for treating hardness of water with their corresponding reagents used.

List-$I$	List-$II$
$A$. Treatment with washing soda	$I$. $Ca(OH)_2$
$B$. Calgon's method	$II$. $Na_2CO_3$
$C$. Clark's method	$III$. $NaAlSiO_4$
$D$. Zeolite/permutit process	$IV$. $Na_6P_6O_{18}$

The correct match is:

Temporary hardness may be removed from water by adding

Consider the following statements regarding heavy water $(D_2O)$. Which of the following statements is/are correct?
$(A)$ Heavy water is used as a moderator in nuclear reactors.
$(B)$ The power of association is higher in $D_2O$ than in ordinary water $(H_2O)$.
$(C)$ Heavy water is more effective as a solvent than ordinary water.

Write two reactions to explain the amphoteric nature of water.