Deionised water is obtained by passing hard water through

The correct statements among the following are

(A) $\mathrm{BeH}_2$ and $\mathrm{MgH}_2$ are polymeric in nature.

(B) LiH is unreactive to oxygen, at moderate temperatures.

(C) $\mathrm{BeH}_2$ and $\mathrm{MgH}_2$ possesses significant covalent character.

(D) The stability of alkali hydrides follows the order $\mathrm{LiH}<\mathrm{NaH}< \mathrm{KH}<\mathrm{RbH}<\mathrm{CsH}$.

The freezing point of heavy water at 1 atm pressure is

The most effective water softening method is