Deionised water is obtained by passing hard water through

Unknown inorganic compound ' $A$ ' is used for water softening. ' $A$ ' reacts with $\mathrm{Na}_2 \mathrm{CO}_3$ to generate an alkaline compound ' $B$ ' whose $\mathrm{pH}=14$. ' $A$ ' on reaction with $\mathrm{CO}_2$ gives cloudy ppt. ' $B$ ' +CaO reacts with unknown organic compound ' $C$ ' to give $\mathrm{C}_6 \mathrm{H}_6$. $A$, $B$ and $C$, respectively, are

Assertion $(\mathrm{A})\left[\mathrm{B}\left(\mathrm{OH}_2\right)_6\right]^{3+}$ and $\left[\mathrm{B}(\mathrm{OH})_4\right]^{-}$form octahedral and tetrahedral structures.

Reason (R) Being electron deficient, boron readily reacts with Lewis bases like $\mathrm{H}_2 \mathrm{O}, \mathrm{OH}^{-}$ The correct option among the following is

The correct order of " $\Delta H_f^{\circ}$ " values of diamond (I), graphite (II) and fullerene (III) is