A comet is observed to move much faster when it is near the Sun than when it is far away. Which one of the following statements gives the best explanation for this observation?
(a) The gravitational force on the comet is stronger at smaller distance, so its kinetic energy increases.
(b) The line joining the comet and the Sun sweeps out equal areas in equal intervals of time.
(c) The Sun exerts an additional tangential force on the comet only at perihelion.
(d) Because the orbital path is shorter near the Sun, the comet must move faster to complete its revolution in the same period.