Assume that random processes x(t) and y(t) are individually and jointly stationary. a. Determine the autocorrelation function of z(t) = x(t) + y(t) in terms of the autocorrelation and cross-correlation functions of x(t) and y(t) b. Determine the autocorrelation function of z(t) when x(t) and y(t) are uncorrelated. c. Determine the autocorrelation function of z(t) when x(t) and y(t) are uncorrelated and have zero means