Jack went running last Saturday morning. He ran the first $12 \mathrm{~km}$ at $12 \mathrm{~km} / \mathrm{h}$ and the second $12 \mathrm{~km}$ at $6 \mathrm{~km} / \mathrm{h}$. Jill ran the same route at a constant speed, and took the same length of time as Jack. Jill's speed in $\mathrm{km} / \mathrm{h}$ was
(A) 8
(B) 9
(C) 6
(D) 12
(E) 24
Jack ran a total of $24 \mathrm{~km}$.

Since he ran the first $12 \mathrm{~km}$ at $12 \mathrm{~km} / \mathrm{h}$, then it took him 1 hour to run the first $12 \mathrm{~km}$.

Since he ran the second $12 \mathrm{~km}$ at $6 \mathrm{~km} / \mathrm{h}$, then it took him 2 hours to run the second $12 \mathrm{~km}$.

Therefore, his run took a total of 3 hours.

Thus, Jill ran $24 \mathrm{~km}$ in 3 hours, so her speed was $8 \mathrm{~km} / \mathrm{h}$.

ANSWER: (A)