Black surfaces emit thermal radiation faster

Steps:

• Hot water in cans loses heat primarily through radiation to the cooler room.
• Surface color affects emissivity: dull black has high emissivity (close to 1), polished silver has low emissivity (close to 0).
• Higher emissivity means faster heat emission from the black can.
• After five minutes, the dull black can's water cools more.

Why A is correct:

• Dull black surface follows Stefan-Boltzmann law with high emissivity, radiating heat ∝ εσT⁴ efficiently (ε ≈ 0.9 vs. 0.05 for silver).

Why the others are wrong:

• B (polished silver): Low emissivity reflects heat, slowing cooling.
• C (dull black): Identical to A, but question specifies A as the cooler one.
• D (polished silver): Same as B, retains heat longer.

Final answer: A