Astronomer: In most cases in which a planet has been detected orbiting a distant star, the planet' s orbit is distinctly oval, whereas the orbits of Earth and several other planets around our sun are approximately circular. However, many comets orbiting our sun have been thrown into oval orbits by close encounters with planets orbiting our sun. So some of the planets in oval orbits around distant stars were probably thrown into those orbits by close encounters with other planets orbiting the same stars. Which one of the following, if true, would most strengthen the astronomer's argument? ['There is no indication that the orbit of any planet orbiting our sun has been affected by a close encounter with another planet orbiting our sun.', 'In most cases in which planets have been discovered orbiting a distant star, more than one planet has been found orbiting the star.', 'When two planets or other large objects in orbit have a close encounter, usually the smaller of the two is the more greatly affected.', "For each distant star that has been found to have a planet, no other object large enough to affect the planet's orbit has been found orbiting the star."]