A pipe is buried in soil with α=4.14 x 10 -7 m2 /s, if the temperature on the surface varies by 50 o C in 6 months, the minimum depth at which the pipe must be buried for temperature variation, over the same period not to exceed 1 o C is (a) 5.0 m (b) 6.7 m (c) 8.8 m (d) 12.5 m (e) 7.4 m (f) None of them