Although Pluto has an atmosphere and is much larger than any asteroid, Pluto is not a true planet. Pluto formed in orbit around the planet Neptune and was then ejected from orbit around Neptune when Triton, Neptune' s largest moon, was captured by Neptune' s gravity. The conclusion of the argument follows logically if which one of the following is assumed? ['If Pluto had not been ejected from its orbit around Neptune, Pluto would not have its current orbit around the sun and would still be a moon.', 'For a celestial body to be a true planet it must have formed in orbit around the sun exclusively.', 'No celestial body can simultaneously be a moon and a planet.', 'Not all celestial bodies that have an atmosphere and orbit the sun are true planets.']