A lake surface is exposed to an atmosphere where the temperature is `lt 0^(@)C`. If the thickness of the ice layer formed on the surface grows form `2cm` to `4cm` in `1` hour. The atmospheric temperature, `T_(a)` will be- (Thermal conductivity of ice `K = 4 xx 10^(-3) cal//cm//s//.^(@)C`, density of ice `= 0.9 gm//ice.` Latent heat of fustion of ice `= 80 cal//m`. Neglect the change of density during the state change. Assume that the water below the ice has `0^(@)` temperature every where)

Options

A. `-20^(@)C`
B. `0^(@)C`
C. `-30^(@)C`
D. `-15^(@)C`

Answer

C