<image>Each of the 4 vertices and 6 edges of a tetrahedron is labelled with one of the numbers $1,2,3,4,5,6,7,8,9$ and 11. (The number 10 is left out). Each number is only used once. The number on each edge is the sum of the numbers on the two vertices which are connected by that edge. The edge $A B$ has the number 9. With which number is the edge $C D$ labelled?