Agas has a volume of 5.0 l at a pressure of 50 kpa. what happens to the volume when temperature is held constant and the pressure is increased to 125 kpa?
a. 12.5 l
b. 0.2 l
c. 2.0 l
d. 1250.0 l

This question is answered using ideal gas model.
At constant temperature you can state that pV is constant. This is the p and V are inversely related. Then if the pressure doubles the volumen will be half. In this case:
 (pV)1 = (pV)2 => V1 = (pV)2 / V2 = 50kPa*5.0L / 125kPa = 2.
 Then the answer is the option c. 2.0 L