The instantaneous frequency of the signal x(t) = 10 cos (100sin(4πt) + 1000πt + 0.3π) in Hz is given by
a. fi(t)=500
b. fi(t)=500+100sin(4πt)
c. fi(t)=500+400πcos(4πt)
d. fi(t) = 500 + 200 cos (4πt)