Le chatelier, pressure and volume

[Bengo]

This question has been bugging me and the more I think about it the more confused I get.

N2O4 ⇔2NO2

Question: the reaction will shift to the right with all of the following changes except

A. Addition of N2O4
B. an increase in volume at constant pressure
C. A decrease in pressure at constant volume
D. Addition of helium gas to the system at constant volume

I know the answer is D but choice B and C confuse me because with pressure and volume, how can you change one without the other if the reaction is already at equilibrium? I can see if more reactants or products are introduced but the question doesn't say that.

Thank you

 

[Yanick]

Think what temperature would do to the system and how you can try to control all the variable taking into account the temperature of the system.

 

[Bengo]

Well my book says you can use a piston to keep pressure constant while changing the volume but I just don't understand how that's possible in this particular question when nothing else is being done to the system.

 

[Yanick]

I'm out of practice with these questions but how will you change the volume inside a container (assumed by displacement of a piston) while keeping the pressure, temp and number of mols constant?

Something's got to give, no?

Also keep in mind that in these types of thermodynamics questions you occasionally need to forget the world and let the math do the talking. Are you familiar with the use of a reaction quotient and partial pressures when analyzing these problems?