When orthoboric acid $({H_3}B{O_3})$ is heated, the residue left is

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 reactions can be used to prepare diborane

$I.\,\,NaBH_4 + BF_3$ (in ether) $\to $

$II.\,\, NaBH_4 + I_2 \to $

$III.\,\, BF_3 + NaH \to $

Hoop's process is used for the purification of the metal