A newly discovered painting seems to be the work of one of two seventeenth-century artists, either the northern German Johannes Drechen or the Frenchman Louis Birelle, who sometimes painted in the same style as Drechen. Analysis of the carved picture frame, which has been identified as the painting's original seventeenth-century frame, showed that it is made of wood found widely in northern Germany at the time, but rare in the part of France where Birelle lived. This shows that the painting is most likely the work of Drechen. Which of the following is an assumption that the argument requires? ['Sometimes a painting so resembles others of its era that no expert is able to confidently decide who painted it.', 'Drechen is unlikely to have ever visited the home region of Birelle in France.', 'The carving style of the picture frame is not typical of any specific region of Europe.', 'The frame was made from wood local to the region where the picture was painted.']