The crystalline form of borax has

($A$) tetranuclear $\left[\mathrm{B}_4 \mathrm{O}_5(\mathrm{OH})_4\right]^{2-}$ unit

($B$) all boron atoms in the same plane

($C$) equal number of $s p^2$ and $s p^3$ hybridized boron atoms

($D$) one terminal hydroxide per boron atom

Which of the following doesn't exist

Which of the following statements about the interstitial compounds is incorrect?

Below are two statements, one labeled as Assertion $(A)$ and the other as Reason $(R):$

Assertion $(A)$: Among group $13$ elements, boron's melting point is unusually high $(2453 \mathrm{~K})$.

Reason $(R):$ Solid boron has a strong crystalline lattice.

In the context of the above statements, choose the correct answer from the following options:

The electrochemical extraction of aluminum from bauxite ore involves.

$(A)$ the reaction of $Al _2 O _3$ with coke ($C$) at a temperature $>2500^{\circ} C$.

$(B)$ the neutralization of aluminate solution by passing $CO _2$ gas to precipitate hydrated alumina $\left( Al _2 O _3 .3 H _2 O \right)$

$(C)$ the dissolution of $Al _2 O _3$ in hot aqueous $NaOH$.

$(D)$ the electrolysis of $Al _2 O _3$ mixed with $Na _3 AlF _6$ to give $Al$ and $CO _2$.

$B{X_3} + N{H_3}\ \xrightarrow {R.T}\ B{X_3}.N{H_3} +$ Heat  of  adduct formation $\left( {\Delta H} \right)$  The numerical value of $\Delta H$ is found to be maximum for