A child riding on a sledge moves down a hill with acceleration $a$. The hill is inclined at an angle $\theta$ to the horizontal. The combined mass of the child and the sledge is $m$. The acceleration due to free fall is $g$. What is the friction force $F$?
- A$m(g\cos\theta - a)$
- B$m(g\cos\theta + a)$
- C$m(g\sin\theta - a)$
- D$m(g\sin\theta + a)$