Based on your observations of the behavior of your computer model of a planet orbiting a star, and on your reading in the textbook, which of the following statements about an elliptical orbit are true?
a. The direction of the planet's momentum is changing every instant. b. The magnitude of the planet's momentum is constant
c. At any instant, the momentum of the planet is tangent to the plant's trajectory
d. The gravitational force on the planet due to the star always acts at a right angle to the planet's momentum
e. At every instant, d/dt points from the planet to the star on Mars