Chad is putting a frame around a picture. He needs to decide whether he wants the frame to be black or brown. He also needs to decide whether he wants the frame to be made of metal or of wood. Finally, he needs to decide whether he wants the picture covered with glass or with plastic. He makes a list of his options as shown. ​
​ {black, metal, glass}
​ {black, metal, plastic}
​{black, wood, glass}
{black, wood, plastic}
​ {brown, metal, plastic}
​ {brown, wood, glass}
​ {brown, wood, plastic}
​
​Which option did Chad forget to include in his list?