In cellular systems, what does the term 'co-channel refer to? a) communication channels that use different frequencies in different cells b) communication channels that use the same frequency in different cells. c) communication channels that co-exist in a given cell and use different frequencies. d) communication channels that co-exist in a given cell and use the same frequencies e) both a and b (but not c and d). f) both c and d (but not a and b). g) none of the above