Jack went running last Saturday morning. He ran the first 12\:km at 12\:km/h and the second 12\:km at 6\:km/h. Jill ran the same route at a constant speed, and took the same length of time as Jack. Jill’s speed in km/h was
Answer Choices
A. 8
B. 9
C. 6
D. 12
E. 24