Black surfaces emit thermal radiation faster than shiny ones

Steps:

• Hot water in cans cools mainly by radiating infrared heat to the cooler room.
• Dull black surfaces have high emissivity, efficiently emitting radiation.
• Polished silver surfaces have low emissivity, reflecting heat and retaining warmth.
• After five minutes, the black can loses more heat, cooling its water faster.

Why A is correct:

• Dull black follows the Stefan-Boltzmann law, where power radiated is proportional to emissivity times T^4; high emissivity means faster cooling.

Why the others are wrong:

• B (polished silver): Low emissivity traps heat, so water stays warmer.
• C (dull black): Duplicate of A, but question specifies A as the coolest can.
• D (polished silver): Same as B, poor radiator keeps water hotter.

Final answer: A