In the obtuse triangle ABC, AM=MB,MD⊥BC,EC⊥BC. If the area of △ABC is 24, then the area of △BED is[asy]
size(200);
defaultpen(linewidth(0.8)+fontsize(11pt));
pair A = (6.5,3.2), B = origin, C = (5.0), D = (3.3,0);
pair Xc = (C.x,4), Xd = (D.x,4), E = intersectionpoint(A--B,C--Xc), M = intersectionpoint(D--Xd, A--B);
draw(C--A--B--C--E--D--M);
label("A",A,NE);
label("B",B,W);
label("C",C,SE);
label("D",D,S);
label("E",E,N);
label("M",M,N);
draw(rightanglemark(D,C,E,7)^^rightanglemark(B,D,M,7));
[/asy]<spanclass=′latex−bold′>(A)</span>9<spanclass=′latex−bold′>(B)</span>12<spanclass=′latex−bold′>(C)</span>15<spanclass=′latex−bold′>(D)</span>18<spanclass=′latex−bold′>(E)</span>not uniquely determined