A bicycle trip is 30 \mathrm{~km} long. Ari rides at an average speed of 20 \mathrm{~km} / \mathrm{h}. Bri rides at an average speed of 15 \mathrm{~km} / \mathrm{h}. If Ari and Bri begin at the same time, how many minutes after Ari finishes the trip will Bri finish?
Answer Choices
A. 50 min
B. 40 min
C. 30 min
D. 20 min
E. 10 min