Amorphous Boron obtained by the following steps $\text{Borax}\stackrel{\left(\text{A}\right)}{\to }{\mathrm{H}}_3\mathrm{B}{\mathrm{O}}_3\underset{\mathrm{Hot}}{\overset{\mathrm{Red}}{\to }}{\mathrm{B}}_2{\mathrm{O}}_3\stackrel{\left(\mathrm{B}\right)}{\to }\mathrm{Boron}$.  What are A and B

• A

  ${\mathrm{H}}_2{\mathrm{SO}}_4,\mathrm{Al}$

• B

  HCl and Carbon

• C

  HCl, Al

• D

  H₂SO₄ and Mg