A constant volume gas thermometer shows pressure readings of `50 cm` and `90 cm` of mercury at `0^(@)C` and `100^(@)C` respectively, The temperature of the bath when pressure reading is `60 cm` of mercury.
A. `25^(@)C`
B. `40^(@)C`
C. `15^(@)C`
D. `12.5^(@)C`