Determine the maximum frequency ${}_B($if it exists$)$ in the spectrum of the following continuous$-$time signals.

a$)x_1(t)=22\mathrm{.}0e^{-80t}u(t)$

b$)x_2(t)=4\mathrm{.}0+2\mathrm{.}2cos(380t)+1\mathrm{.}4cos(640t)$

c$)x_3(t)={\displaystyle \underset{k=-100}{\overset{100}{}}}\frac{1}{j8k}{e}^{j150kt}$