The coefficient of linear expansion of crystal in one direction is \[{{\alpha }_{1}}\] and that in every direction perpendicular to it is \[{{\alpha }_{2}}\]. The coefficient of cubical expansion is

• A.            \[{{\alpha }_{1}}+{{\alpha }_{2}}\]                                  
• B.            \[2{{\alpha }_{1}}+{{\alpha }_{2}}\]
• C.            \[{{\alpha }_{1}}+2{{\alpha }_{2}}\]                               
• D.            None of these