An anisotropic material has coefficients of linear thermal expansion $\alpha_{1}, \alpha_{2}$ and $\alpha_{3}$ along $x, y$ and $z$ -axis respectively. Coefficient of cubical expansion of its material will be equal to
(a) $\alpha_{1}+\alpha_{2}+\alpha_{3}$
(b) $\alpha_{1}+2 \alpha_{2}+3 \alpha_{3}$
(c) $3 \alpha_{1}+2 \alpha_{2}+\alpha_{3}$
(d) $\frac{\alpha_{1}+\alpha_{2}+\alpha_{3}}{3}$