The coefficient of linear expansion of crystal in one direction is  ${\mathrm{\alpha }}_1$ and that in every direction perpendicular to it is ${\mathrm{\alpha }}_2$ . The coefficient of cubical expansion is

• A

  ${\mathrm{\alpha }}_1+{\mathrm{\alpha }}_2$

• B

  $2{\mathrm{\alpha }}_1+{\mathrm{\alpha }}_2$

• C

  ${\mathrm{\alpha }}_1+2{\mathrm{\alpha }}_2$

• D

  None  of these