$[15]$ Determine the Nyquist sampling rate and the Nyquist sampling interval $($period$)$ for the following signals: a$)$ sin $(40$t$)$sin $(60$t$)/22$ b$)1+$ cos$(2,000)+$ sin $(4,000)$ c$)$ The signal $2(),$ where $()$ is a signal bandlimited to $100$ H