Kepler's third law states that the farther a planet is from the Sun, the slower it moves in its orbit.

• A. The farther a planet is from the Sun, the slower it moves in its orbit.
• B. The faster a planet is when it is far from the Sun.
• C. Orbital speed is the same for all planets.
• D. Distance does not affect orbital period.

Answer: (A)

This is about how distance from the Sun governs orbital speed and how long a planet takes to complete an orbit. Gravity sets the relationship: in a circular orbit, the speed v satisfies v = sqrt(GM/r). So as the distance r from the Sun increases, the orbital speed decreases. Kepler’s third law ties distance to the orbital period: P^2 is proportional to a^3 (where a is the orbit’s size). Planets farther from the Sun have longer orbital periods, meaning they complete orbits more slowly on average. So the statement that the farther a planet is, the slower it moves in its orbit is a correct qualitative summary of these ideas.

The other ideas conflict with gravity and Kepler’s law: speed isn’t greater when the planet is farther out, orbital speed isn’t the same for all planets, and distance does affect the orbital period.