What does Kepler's first law of planetary motion imply?
A. A planet moves at a constant speed at all points in its orbit. B. Uniform circular motion is adequate to describe the motion of all planets. C. The distance between the planet and the sun changes as the planet orbits the sun. D. The distance that a planet is from the sun depends on the mass of the planet. E. Planets closer to the sun than Earth will show retrograde motion