A cylinder of radius R is surrounded by a cylindrical shell of inner radius R and outer radius 2R. The thermal conductivity of the material of the inner cylinder is K_{1} and that of the outer cylinder is K_{2}. Assuming no loss of heat, the effective thermal conductivity of the system for heat flowing along the length of the cylinder is:
a. 
K_{1 }+ K_{2} 
b. 

c. 

d. 

A cylinder of radius R is surrounded by a cylindrical shell of inner radius R and outer radius 2R. The thermal conductivity of the material of the inner cylinder is K_{1} and that of the outer cylinder is K_{2}. Assuming no loss of heat, the effective thermal conductivity of the system for heat flowing along the length of the cylinder is:
a. 
K_{1 }+ K_{2} 
b. 

c. 

d. 
