Cooling air increases its density through contraction

Steps:

• Cooling unit extracts heat from nearby air, lowering its temperature.
• Reduced temperature decreases molecular kinetic energy.
• Molecules move closer together, contracting the air's volume.
• At constant mass, smaller volume means higher density.

Why D is correct:

• Density = mass/volume; cooling contracts air (decreases volume) while mass stays constant, so density increases per ideal gas law (PV = nRT, where lower T reduces V at fixed P).

Why the others are wrong:

• A: Molecules do not become smaller; density actually increases, not decreases.
• B: Molecules do not become smaller, despite correct density increase.
• C: Molecules do move closer, but this increases density, not decreases it.

Final answer: D