$($t$).5.$ Determine the Nyquist sampling frequencies in Hz needed to sample the following analog signals without aliasing error. $($a$)$ Arbitrary signal f$($t$)$ with bandwidth $40$ kHz$.($b$)$ f$($t$)=$ sinc$(40$t$).($c$)$ g$($t$)=$ sinc$(100$t$)+$ sinc$(40$t$)$ cos $(20$