On heating, borax first swells up by losing water molecules. On further heating it changes to transparent liquid which solidifies to glass like material known as borax bead. In the following equation, supply the formulae of compounds. A and B respectively.

${\mathrm{Na}}_2{ \mathrm{B}}_4{\mathrm{O}}_7·10{\mathrm{H}}_2\mathrm{O}⟶{\mathrm{Na}}_2{ \mathrm{B}}_4{\mathrm{O}}_7\to \mathrm{A}+\mathrm{B}$

• A

  Na₂B₂O₄,B₂O₄

• B

  NaBO₂,B₂O₃

• C

  Na₂B₂O₄,B₄O₆

• D

  NaBO₂,B₄O₆