Borax is actually made of two tetrahedra and two triangular units joined together and should be written as : $Na_2[B_4O_5(OH)_4]·8H_2O$

Consider the following statements about borax :

$a.$  Each boron atom has four $B-O$ bonds

$b.$  Each boron atom has three $B-O$ bonds

$c.$  Two boron atoms have four $B-O$ bonds while other two have three $B-O$ bonds

$d.$  Each boron atom has one $-OH$ groups

Select correct statement(s) :

• A

  $a, b$

• B

  $b, c$

• C

  $c, d$

• D

  $a, c$