A liquid when heated in a copper vessel and when heated in a silver vessel, the apparent coefficient of expansion is ' $C^{\prime}$ and ' $S^{\prime}$, respectively. If coefficients of linear expansion of copper is ' $A^{\prime}$, then coefficient of linear expansion of silver is :
(a) $\frac{C+S-3 A}{3}$
(b) $\frac{C+3 A-S}{3}$
(c) $\frac{3 A-S-C}{3}$
(d) $\frac{C+S+3 A}{3}$