The volume of block of metal changes by $0.12 \%$ when heated through $20^{\circ} \mathrm{C}$ then $\alpha$ is :
(a) $2.0 \times 10^{-5}\left({ }^{\circ} \mathrm{C}\right)^{-1}$
(b) $4.0 \times 10^{-5}\left({ }^{\circ} \mathrm{C}\right)^{-1}$
(c) $6.0 \times 10^{-5}\left({ }^{\circ} \mathrm{C}\right)^{-1}$
(d) $8.0 \times 10^{-5}\left({ }^{\circ} \mathrm{C}\right)^{-1}$