Incredible is to believable as disinterested is to

Disinterested does not mean uninterested; rather, disinterested means impartial or unbiased.

Incredible means not credible, that is, not believable. So incredible is the opposite of believable. To complete the analogy posed in the question, we need to supply an opposite of disinterested. So I'll go with partial as my answer:

Incredible is to believable as disinterested is to partial.

Is partial an answer choice? If not, pick the choice most similar to partial.