A solid rectangular sheet has two different coefficients of linear expansion \(\alpha_{1}\) and \(\alpha_{2}\) along its length and breadth respectively. The coefficient of surface expansion is (for \(\alpha_{1} t<<1, \alpha_{2} t<<1\) )
(A) \(\frac{\alpha_{1}+\alpha_{2}}{2}\)
(B) \(2\left(\alpha_{1}+\alpha_{2}\right)\)
(C) \(\frac{4 \alpha_{1} \alpha_{2}}{\alpha_{1}+\alpha_{2}}\)
(D) \(\alpha_{1}+\alpha_{2}\)
