JEE Main 2019 Physics question

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 and that of the outer cylinder is . Assuming no loss of heat, the effective thermal conductivity of the system for heat flowing along the length of the cylinder is:

• (A)
• (B)Correct
• (C)
• (D)