Which of the following effects will make the ratio (PV) / (nRT) less than 1 for a real gas?

Question

a) The gas molecules are large enough to occupy a substantial amount of space.
b) A large number of molecules have speeds greater than the average speed.
c) The gas molecules have a very low molecular weight.
d) The gas molecules attract one another.
e) The gas molecules repel each other.

Answer 1

I think most of this covered in discussions of the "imperfect gas" and the van der Waals equation.