Answer:Clay
Explanation: clay by nature is not as strong as the others mentioned in the alternative and it can only be worked on when soft and form desired shape when dry because it is malleable making it an unlikely carving tool
Annunciation Triptych (Merode Altarpiece), Workshop of Robert Campin (Netherlandish, ca. Due to rights restrictions, ... Classification: Paintings-Panels.
They are Cubist because they show objects from many views.