Which of the following methods is used to remove hardness from water?

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.

Which of the following is not correct for $D_2O$?

What is used to soften water by the Clark's method?

The temporary hardness of a water sample is due to compound $X$. Boiling this sample converts $X$ to compound $Y$. $X$ and $Y$,respectively,are

The chemical name of calgon is