Which salt does not produce hardness in water?

Hard water contains ions of

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.

Explain the importance of water for living organisms.

Polyphosphates are used to soften water because they:

Describe the methods to remove the permanent hardness of water.