Two rods of length `d_(1)` and `d_(2)` and coefficients of thermal conductivites `K_(1)` and `K_(2)` are kept touching each other. Both have the same area of cross-section. The equivalent thermal conductivity.

Options

A. `((d_(1)+d_(2))(K_(1)d_(2)+K_(2)d_(1)))/(2(K_(1)+K_(2)))`
B. `((d_(1)-d_(2))(K_(1)d_(2)+K_(2)d_(1)))/(2(K_(1)+K_(2)))`
C. `(K_(1)d_(1)+K_(2)d_(2))/(d_(1)+d_(2))`
D. `(K_(1)+K_(2))/(d_(1)+d_(2))`