An asteroid falling towards a planet has a speed of 20.0km s−1 at point P. Ten minutes later, the asteroid is at point Q and its speed is 20.6km s−1. What is the average gravitational field strength between P and Q?
(A) 0.001N kg−1
(B) 0.006N kg−1
(C) 1N kg−1
(D) 6N kg−1