Geometry: Common Core (15th Edition)

$B$
We can set up the proportion as follows: $\frac{1 in.}{5 mi.} = \frac{9 in.}{d}$ This corresponds to option $A$, so we can exclude this option as the answer. A property of proportions is that we can invert the two ratios and still have an equivalent proportion: $\frac{5 mi.}{1 in.} = \frac{d}{9 in.}$ This corresponds to option $D$, so we can also exclude this option as the answer. If we invert the ratios and move them from one side of the equation to the other, we would still have an equivalent proportion; therefore, we can also exclude option $C$ as the answer. The only option that remains is option $B$. There is no property of proportions that tells us that we can switch the means and the extremes and still have an equivalent proportion.