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

a) $\displaystyle \frac {a}{f} = \frac{b}{c}$

b) $\displaystyle \frac {f}{c} = \frac{b}{a}$

c) $\displaystyle \frac {c}{a} = \frac{f}{b}$

d) $\displaystyle \frac {a}{c} = \frac{b}{f}$

e) $\displaystyle \frac {af}{bc} = 1$

Can someone please show me the steps? Does nonzero numbers simply mean numbers that can't equal to zero?

Thanks in advance