If the lengths of the sides of a rectangular parallelopiped are 3,2,1 then the angle between two diagonals out of four diagonals is

• A

  ${\cos }^{-1}\left(\frac{{\displaystyle 6}}{{\displaystyle 7}}\right)$

• B

  ${\cos }^{-1}\left(\frac{{\displaystyle 2}}{{\displaystyle 3}}\right)$

• C

  ${\cos }^{-1}\left(\frac{{\displaystyle 13}}{{\displaystyle 14}}\right)$

• D

  ${\cos }^{-1}\left(\frac{{\displaystyle 9}}{{\displaystyle 14}}\right)$