Does the unit rate always have to be set up as the denominator?

Yes. It always has to be /1.

Yes, as if you said your car was going 50 miles for 2 hours, and did this:
2 ÷ 50
50 ÷ 50,
it would equal 0.04 miles per hour. That is clearly incorrect. But, if you did:
50 ÷ 2
2 ÷ 2,
it would equal 25 miles an hour. Conclusion, you must have the unit rate  set up as the denominator

Bis the right answer