A moon is in an elliptical orbit about a planet. At point A, the moon is farther from the planet and has a speed of u_A and a distance of R_A from the planet. At point B, the moon is closer to the planet and has a speed of u_B. Which of the following explains a correct method for determining the distance of the moon from the planet at point B in terms of the given quantities?
A. Conservation of angular momentum because Fg is same for both
B. Conservation of angular momentum because Fg on moon is always toward planet
C. Conservation of energy because Fg is always directed towards planet
D. Conservation of energy, because Fg is the same for both
My teacher's key says the answer is B, but I have no idea why.