9. Although the Sun is at only one of the two foci of its elliptical orbit, an object like Halley's comet 'turns around' at both ends of its very elongated orbit. This is: A. because of the combined gravitational attraction of all the planets acting together as a complete 'system' B. because it is following a simple, predictable curved path, and so it 'falls back' even though it is completely beyond the Sun's gravitational influence at the far end of its orbit C. because of the dominant pull of the Sun's gravity in both cases D. because it slows down as it moves through the thin interplanetary gas that permeates the Solar System