Which of the following statements is an effect of compressing gas molecules at constant temperature? 

A.

The pressure on the molecules decreases.

B.

The molecules gain more kinetic energy

C.

The molecules make more impact per second on the walls of the container

D.

The molecules increase in size

Correct answer is C

More collisions mean more force, so the pressure will increase. 

When the volume decreases, the pressure increases.

This shows that the pressure of a gas is inversely proportional to its volume.

This is shown by  Boyle's law.