Borax contains tetra nuclear unit [B₄O₅ (OH)₄]^2- of 48 atoms. The correct formula of borax is:

• A

  ${\text{Na}}_2\left[ {\text{B}}_4{\text{O}}_5{\left(\text{OH}\right)}_4\right]\cdot 8{\text{H}}_2\text{O}$

• B

  ${\text{a}}_2\left[ {\text{B}}_4{\text{O}}_5{\left(\text{OH}\right)}_4\right]\cdot 6{\text{H}}_2\text{O}$

• C

  ${\text{Na}}_2\left[ {\text{B}}_4{\text{O}}_5{\left(\text{OH}\right)}_4\right]\cdot 5{\text{H}}_2\text{O}$

• D

  ${\text{Na}}_2\left[ {\text{B}}_4{\text{O}}_5{\left(\text{OH}\right)}_4\right]\cdot 7{\text{H}}_2\text{O}$