Two metal wires of idential dimensions are connected in series. If `sigma_(1) and sigma_(2)` are the conductivities of the metals respectively, the effective conductivity of the combination is

Options

A. `sigma_(1)+ sigma_(2)`
B. `(sigma_(1) + sigma_(2))/(2)`
C. `sqrt(sigma_(1) sigma_(2))`
D. `(2 sigma_(1) sigma_(2))/(sigma_(1) + sigma_(2))`