At $20^{\circ} \mathrm{C}$, a steel ruler of $20 \mathrm{~cm}$ long is graduated to give correct reading, but when it is used at a temperature of $40^{\circ} \mathrm{C}$, what will be the actual length of the steel ruler? $\left[\alpha_{\text {stèel }}=1.2 \times 10^{-5}\left({ }^{\circ} \mathrm{C}\right)^{-1}\right]$
(a) $22.02 \mathrm{~cm}$
(b) $19.6 \mathrm{~cm}$
(c) $20.0048 \mathrm{~cm}$
(d) $18.0002 \mathrm{~cm}$