What exactly causes an increase in resistivity? 
Explain in terms of charge carriers why the resistance of copper is less than that of CuSO4 solution, which is less than that of hydrogen gas

This is the question directly asked for homework, and I'm not quite sure if I have got the right end of the stick, nor how to answer it. I understand that 'charge carriers' in metals are delocalised electrons, positive and negative ions in salt solutions and ions & electrons in gases when they are made into plasmas by heating. Is the increase in resistivity down to size of charge carriers, and/or how much they are vibrating due to temperature (i.e. for gases to conduct they must be very hot)?
I know this question is a bit stupid but neither my teacher nor the textbook are very helpful, and I would really appreciate any help.
Many thanks!
 A: Here are some ideas, which you can combine with some of yours and make a short essay. 
Electrical resistance, in general, is caused by the collisions of the electric charge carriers in materials as they are moving from one electrode to the other.  Also the type of electric charge carriers is important. In copper wire the electric charge carriers are very tiny particles called electrons, while in CuSO$_4$ solution the carriers are not only electrons but also ions, which are very much bigger than the electros in copper wire.  
Also, the density of electric charge carriers is very high in the copper wire than in CuSO$_4$ or in gasses. The chance of electrons colliding with ions in the copper wire is much less than that of the big ions colliding as they move from one end to the other in the CuSO$_4$ solution.
In gasses at low temperature, as you say, there are no ions so there are no charges to move and generate an electric current, so effectively, the electrical resistance of gasses at low temperature is extremely large. One needs very high voltage to start an electric current in gasses at low temperature and pressure (pressure is also importnat for gass conductivity), as the latter need to be ionised first (this phenomenon is called gas discharge). At higher temperature where atoms become ionised, there are electric charge carriers and the electrical conductivity increases (resistance drops.)
