Assume that Z(t) = X(t) + Y(t), where X(t) and Y(t) are jointly stationaryrandom processes.

A、Z(t)isastationaryprocess;
B、Sz(f)=Sx(f)+Sy(f);
C、Sz(f)=Sx(f)+Sy(f)+Sxy(f)+Syx(f);
D、IfthetwoprocessesX(t)andY(t)areuncorelatedandatleastoneoftheprocessesiszeromean，thenSz(f)=Sx(f)+Sy(f)