If two metallic plates of equal thickness and thermal conductivities `K_1` and `K_2` ​are put together face to face and a common plate is constructed, then the equivalent thermal conductivity of this plate will be

Options

A. `\frac { K _ { 1 } K _ { 2 } } { K _ { 1 } + K _ { 2 } }`
B. `\frac { 2K _ { 1 } K _ { 2 } } { K _ { 1 } + K _ { 2 } }`
C. `\frac { ( K _ { 1 } ^ { 2 } + K _ { 2 } ^ { 2 } ) ^ { 3 / 2 } } { K _ { 1 } K _ { 2 } }`
D. `\frac { ( K _ { 1 } ^ { 2 } + K _ { 2 } ^ { 2 } ) ^ { 3 / 2 } } { 2 K _ { 1 } K _ { 2 } }`

Answer

B