For a discrete-time signal y(n) = ej(πn/2 + π/2), which one of the following statements is true?

The Fourier transform of a unit impulse function δ(t) is:

The region of convergence (ROC) of the Z-transform of the signal x(n) = 2n u(n) is:

Autocorrelation of a signal measures the similarity of the signal with ____________.