Boric acid on heating at 100°C gives (X). X on heating at 160°C gives (Y) and Y. 
On heating at red hot gives B₂O₃ then X and Y are

• A

  ${\mathrm{H}}_2{\mathrm{B}}_6{\mathrm{O}}_9,{\mathrm{H}}_2{\mathrm{B}}_4{\mathrm{O}}_7$

• B

  ${\mathrm{HBO}}_2,{\mathrm{H}}_2{\mathrm{B}}_4{\mathrm{O}}_7$

• C

  ${\mathrm{H}}_2{\mathrm{B}}_4{\mathrm{O}}_7,{\mathrm{HBO}}_2$

• D

  ${\mathrm{H}}_2{\mathrm{B}}_4{\mathrm{O}}_7,{\mathrm{H}}_2{\mathrm{B}}_6{\mathrm{O}}_9$