Which rate below is a unit rate?
A: 1 mile/ 5 minutes
B: 52 miles/ 2 gallons
C: $250/ 40 hours
D: 3 cups/ 1 scoop
A: 1 mile/ 5 minutes
B: 52 miles/ 2 gallons
C: $250/ 40 hours
D: 3 cups/ 1 scoop