Cooling reduces molecular speed via kinetic energy

Steps:

• Cooling lowers the gas temperature.
• Temperature measures average kinetic energy of gas molecules.
• Kinetic energy decreases with lower temperature.
• Decreased kinetic energy means molecules move more slowly.

Why D is correct:

• Kinetic molecular theory states average kinetic energy equals (3/2)kT, so cooling (lower T) slows molecular motion.

Why the others are wrong:

• A: Slower molecules collide less often with walls.
• B: Gas molecules are point particles and do not contract.
• C: Constant volume keeps molecular spacing unchanged.

Final answer: D