Which of the following statements is correct for graph multiplication method?
选项：

A:The displacement of any type of structures subjected to external loads can be obtained by graph multiplication method.;
B:When EI=constant, graph multiplication method can be used for arch structures.;
C:When graph multiplication method is used, the ordinate yc in the formula is the centroid ordinate of the straight-line bending moment diagram.;
D:When graph multiplication method is used, if both bending moment diagrams are straight-line diagrams, the ordinate yc can be taken from either diagram.