单项选择题
A.C1-y1+C2y2+y3 B.C1y1+C2y2-(C1+C2)y3 C.C1y1+C2y2-(1-C1-C2)y2 D.C1y1+C2y2+(1-C1-C2)y3
A.E(X)=E(Y) B.E(X2)-[E(X)]2=E(Y2)-[E(Y)]2 C.E(X2)=E(Y2) D.E(X2)+[E(X)]2=E(Y2)+[E(Y)]2