A crystal has a coefficient of linear expansion $\;13\times 10^{-7}/^{0}C\;$ , $\;12 \times 10^{-7}/^{0}C\;$ and $\;10 \times 10^{-7}/^{0}C\;$ in three mutually perpendicular directions respectively . Then the coefficient of cubical expansion of the crystal is

Question

$(a)\;12.5\times 10^{-7}/^{0}C\qquad(b)\;25\times 10^{-7}/^{0}C\qquad(c)\;35\times 10^{-7}/^{0}C\qquad(d)\;17.5\times 10^{-7}/^{0}C$