Charlotte is training to run a marathon. She just finished a 12 mile run. That distance is 6 miles farther than 3 times the distance she could run when she started training. The equation 3d + 6 = 12 represents this situation.
How far could Charlotte run when she started training?
A. 6 miles
B. 1.5 miles
B. 2.5 miles
D. 2 miles