In an anisotropic medium, the coefficients of linear expansion of a solid are $\alpha_{1}, \alpha_{2}$ and $\alpha_{3}$ in three mutually perpendicular directions. The coefficient of volume expansion for the solid is
(a) $\alpha_{1}-\alpha_{2}+\alpha_{3}$
(b) $\frac{\alpha_{1}+\alpha_{2}+\alpha_{3}}{3}$
(c) $\alpha_{1}+\alpha_{2}+\alpha_{3}$
(d) none of these