A and C, my navigational / positional equipment consists of:
ASI. .P11 Compass. .Altimeter
Oh yes, and a PilotIII GPS
Unless the Mode S transponder manufacturers come out with a unit that a handheld GPS can connect to then 99.9% of the GA fleet is not going to be able to supply the data the unit requires. Alternatively, if they come up with a unit that has a built-in GPS and a colour moving map, more people might take less convincing that it is a good idea.