One of the effects of lead poisoning is an inflammation of the optic nerve, which causes those who have it to see bright haloes around light sources. In order to produce the striking yellow effects in his "Sunflowers" paintings, Van Gogh used Naples yellow, a pigment containing lead. Since in his later paintings, Van Gogh painted bright haloes around the stars and sun, it is likely that he was suffering from lead poisoning caused by ingesting the pigments he used. Which one of the following is an assumption on which the argument relies? ['Van Gogh continued to use paints containing lead after having painted the "Sunflowers" paintings.', "In Van Gogh's later paintings he painted some things as he saw them.", 'The effects of Naples yellow could not have been achieved using other pigments.', 'Van Gogh did not have symptoms of lead poisoning aside from seeing bright haloes around light sources.']