Measurements of the motion of the planet Uranus seem to show Uranus being tugged by a force pulling it away from the Sun and the inner planets. Neptune and Pluto, the two known planets whose orbits are farther from the Sun than is the orbit of Uranus, do not have enough mass to exert the force that the measurements indicate. Ttherefore, in addition to the known planets, there must be at least one planet in our solar system that we have yet to discover. Which one of the following, if true, most seriously weakens the argument? ['There is a belt of comets beyond the orbit of Pluto with powerful gravitational pull.', 'The force the Sun exerts on Uranus is weaker than the force it exerts on the inner planets.', 'Neither Neptune nor Pluto is as massive as Uranus.', 'Pluto was not discovered until 1930.']