GMAT Critical Reasoning (CR) | Re: Astronomers have long thought that the irregularity

I disagree with the OA as Pluto turns out not to be any reason for Neptune's irregular orbit since the argument states "planet is much too small to exert the amount of gravitational pull on Neptune that astronomers once thought it did".
And the option d indicates that Pluto's pull is not the sole reason to cause the irregularity, but it is one.

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