Amorphous boron on burning in air forms

• A

  $\mathrm{B}{\left(\mathrm{OH}\right)}_3$

• B

  Mixture of ${\mathrm{B}}_2{\mathrm{O}}_3$ and BN

• C

  Only ${\mathrm{B}}_2{\mathrm{O}}_3$

• D

  Only BN