In a 10 m deep lake, the bottom is at a constant temperature of `4^(@)C`. The air temperature is constant at `- 4^(@)C. K_(ice) = 3 K_(omega)`. Neglecting the expansion of water on freezing, the maximum thickness of ice will be
A. `7.5m`
B. `6m`
C. `6m`
D. `2.5m`