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Which is an example of radiation?

An ice pack is placed on a knee, and the knee cools down.
A rock is warmed by an ultraviolet heat lamp.
Hot coffee in a mug causes the mug to warm up.
Cold salt water sinks to the bottom of the ocean.

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Answered by GPT-5 mini AI
A rock is warmed by an ultraviolet heat lamp.

Reason: radiation is energy transfer by electromagnetic waves (no contact or moving fluid needed). The lamp heats the rock by EM radiation. The ice pack/knee and coffee/mug are conduction (direct contact); cold salt water sinking is convection/density-driven flow.