Radiator heats room air through fluid movement

Steps:

• Radiator transfers heat to nearby air molecules.
• Heated air becomes less dense and rises.
• Cooler air sinks to replace it, creating circulation.
• This bulk movement spreads heat throughout the room.

Why B is correct:

• Convection is the heat transfer process in fluids where warmer, less dense material rises and cooler material sinks, driving circulation as defined in thermodynamics.

Why the others are wrong:

• A: Conduction requires direct molecular contact without bulk fluid motion, like heat through a solid wall.
• C: Evaporation involves a phase change from liquid to gas, not applicable to heating air with a radiator.
• D: Expansion refers to volume increase due to temperature but is not a heat transfer mechanism.

Final answer: B