Which of the following statements correctly states Kepler’s second law of planetary motion?
a) The square of the time period of a planet is proportional to the cube of the semi-major axis of its orbit.
b) All planets move in elliptical orbits with the Sun at their centre.
c) The line joining a planet to the Sun sweeps out equal areas in equal intervals of time.
d) The line joining a planet to the Sun sweeps out equal angles in equal intervals of time.