George walks 1 mile to school. He leaves home at the same time each day, walks at a steady speed of 3 miles per hour, and arrives just as school begins. Today he was distracted by the pleasant weather and walked the first \dfrac{1}{2} mile at a speed of only 2 miles per hour. At how many miles per hour must George run the last \dfrac{1}{2} mile in order to arrive just as school begins today?
Answer Choices
A. 4
B. 6
C. 8
D. 10
E. 12