Physics professor: Some scientists claim that superheated plasma in which electrical resistance fails is a factor in causing so-called "ball lightning. " If this were so, then such lightning would emit intense light and, since plasma has gaslike properties, would rise in the air. However, the instances of ball lightning that I observed were of low intensity and floated horizontally before vanishing. Thus, superheated plasma with failed electrical resistance is never a factor in causing ball lightning. The physics professor's conclusion follows logically if which one of the following is assumed? ['All types of ball lightning have the same cause.', 'The phenomena observed by the physics professor were each observed by at least one other person.', 'Ball lightning can occur as the result of several different factors.', 'Superheated plasma in which electrical resistance fails does not cause types of lightning other than ball lightning.']