E(XY)=E(X)E(Y)
D(X-Y)=D(X)-D(Y)
D(X-Y)=D(X)+D(Y)
D(X-Y+3)=D(X-Y)
A选项缺少“X与Y独立”的条件； B选项中D(X-Y)=D(X)+D(Y)-2Cov(X,Y); C选项中D(X+Y)=D(X)+D(Y)+2Cov(X,Y); D选项与方差性质一致
