Passage Summary: Uranus is moving as if something heavy is pulling on it, but since the known nearby planets aren't heavy enough, there must be a secret planet out there.

Conclusion: There must be at least one undiscovered planet in our solar system exerting a gravitational pull on Uranus.

Reasoning: Uranus appears to be pulled by a force away from the Sun, and the known outer planets, Neptune and Pluto, lack the mass to account for this force.

Analysis: To weaken this argument, we need to find a way to explain the 'tug' on Uranus without requiring a new planet. The author assumes the current measurements of Neptune and Pluto's mass are perfectly accurate and that no other celestial phenomena could be responsible. Look for an answer that suggests the data regarding Uranus's motion is flawed, or one that provides an alternative source for the gravitational force, such as a different type of celestial body or a miscalculation of the mass of known planets.