$3.$ Find the Nyquist rate $($minimum sampling rate$)$ for each of the following signals: a$)$ X$($t$)=2$sinc$(10$t$)$ b$)$ x$($t$)=3$sinc$(100$t$)$ c$)$ X$($t$)=4$rect$(10$t$)$ d$)$ x$($t$)=-5$sin$(20$nt$)$cos$(30$nt$)$