Boron is a rare element in Earth's crust. Kernite (or) Razorite is a mineral of Boron whose formula is 

• A

  ${\mathrm{Na}}_2{\mathrm{B}}_4{\mathrm{O}}_7.10{\mathrm{H}}_2\mathrm{O}$

• B

  ${\mathrm{Na}}_2{\mathrm{B}}_4{\mathrm{O}}_7.5{\mathrm{H}}_2\mathrm{O}$

• C

  ${\mathrm{Na}}_2{\mathrm{B}}_4{\mathrm{O}}_7.4{\mathrm{H}}_2\mathrm{O}$

• D

  ${\mathrm{Na}}_2{\mathrm{B}}_4{\mathrm{O}}_7$