Three metal rods made of copper, brass and steel each of area of cross-section \(4 \mathrm{~cm}^2\) are joined as shown in the figure. Their lengths are respectively \(46 \mathrm{~cm}, 13 \mathrm{~cm}\) and \(12 \mathrm{~cm}\). Their coefficients of thermal conductivity are respectively \(0.92,0.26\) and 0.12 , all in CGS units. The rods are thermally insulated from the surroundings except at the ends. Rate of flow of heat through the copper rod, in cal s \({ }^{-1}\) is

Apply the relevant identity from the chapter and substitute the values established in the previous step, keeping the original constraint in view.

Use the simplified expression to isolate the unknown, then plug the result back into the question setup to confirm the working is internally consistent.

Cross-check against a boundary or limiting case. Both the dimensional balance and the qualitative behaviour at the extreme should agree with the candidate answer.

Combine the steps above to identify the correct option. The remaining choices each fail at least one of the consistency, dimensional, or boundary checks.