Which is an example of a proportional relationship?
A. The yogurt shop charges $0.30 per pound.
B. The yogurt shop charges $0.30 per pound plus $1.00 for the cup.
C. John ran 2 miles in 15 minutes then ran 3 miles in 20 minutes.
D. The car traveled 40 mph for half an hour and then traveled 60 mph for 2 hours.