I'd agree with PP's assessment; I doubt the politicos had any real clue what they were doing when they switched from B to C (and back again). If I was to hazard a guess I'd say they were bounced into the switch to C by lobbying, possibly from the Navy, and then bounced back to B once someone did the maths on both Carrier conversion costs and more importantly through life and currency costs.
B is obviously a compromise solution driven by economic factors. If money was no issue we'd likely have some A, some C and maybe some B as well
