Maximum number of atoms which are present in one plane of $H_3BO_3$?

Which is not true about borax?

Why does boron trifluoride behave as a Lewis acid?

Which of following salt does not give borax bead test 

Borax is converted into crystalline boron by the following steps 

${B{orax}}\xrightarrow{X}{H_3}B{O_3}\xrightarrow{\Delta }{B_2}{O_3}\xrightarrow[\Delta ]{Y}B$,

$X$ and $Y$ are respectively

Among the following, the correct statement(s) is(are)

$[A]$ $\mathrm{A} 1\left(\mathrm{CH}_3\right)_3$ has the three-centre two-electron bonds in its dimeric structure

$[B]$ $\mathrm{BH}_3$ has the three-centre two-electron bonds in its dimeric structure

$[\mathrm{C}] \mathrm{AlCl}_3$ has the three-centre two-electron bonds in its dimeric structure

$[D]$ The Lewis acidity of $\mathrm{BCl}_5$ is greater than that of $\mathrm{AlCl}_5$