a$)$ When does a signal not have a Fourier transform?

b$)$ When does the bilateral Laplace transform become the same as the unilateral Laplace transform?

c$)$ What is the Region of Convergence $($ROC$)$ in the Laplace transform? Explain it with an example.

d$)$ When cannot you plugs $=$ jw to obtain Fourier transform from Laplace transform?