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I listened to JY's BR call on this question ( https://7sage.com/discussion/#/discussion/21699 ).
I don't get why he was so dismissive of answer choice C -- I would say it weakens the argument. An alternative explanation as to why the artist's face showed up in the painting (other than him painting it himself) serves to weaken the argument that painter of the painting has been determined. And that's what C provides -- his face may be there because the actual painter used him as a live model. I mean, he's an artist, completely reasonable that he would be in the milieu of artists in the area, and was asked by a friend to pose as a live model.
I agree that D more strongly weakens, but disagree with (what seemed to be) JY's position that C doesn't weaken at all.
Admin note: edited title mildly; please use the format of "PT#.S#.Q# - [brief description]"