Should liters go on the top or the bottom of a conversion factor?

What's with the "or" in between conversion factors? Both conversion factors are equal to 1. However, we want to show that you can write conversion factors two ways—the base unit can be written on the top or bottom as long as the conversion factor is equivalent to 1 in the end.

Explanation:

what experimental evidence led scientists to change from the previous model to this one?

a few of the positive particles aimed at a gold foil seemed to bounce back.

the colors of light emitted from heated atoms had very specific energies.

experiments with water vapor showed that elements combine in specific proportions.

cathode rays were bent in the same way whenever a magnet was brought near them.