If a, b, c, and f are four nonzero numbers, then all of the following proportions are equivalent except:

answer = a\f = b\c

The other options are permutations of this like f\c = b\a or c\a = f\b

Can someone explain why that is true? It says nonzero numbers so under certain circumstances can't I make it true? It seems that the problem doesn't describe enough for me to actually know