Two steel rails each of 12 m length are laid with a gap of 1.5 mm at ends at a temperature of 24°C. The thermal stress produced at a temperature of $$40^\circ C$$ is (take $$E = 2 \times 10^5$$ N/mm$$^2$$ coefficient of thermal expansion = $$12 \times 10^{-6}/^\circ C$$)

A

10.5 N/mm$$^2$$

B

12.5 N/mm$$^2$$

C

13.4 N/mm$$^2$$

D

15.5 N/mm$$^2$$