The isomerization of methyl isonitrile (CH $_{3} \mathrm{NC}$ ) to acetonitrile $\left(\mathrm{CH}_{3} \mathrm{CN}\right)$ was studied in the gas phase at $215^{\circ} \mathrm{C}$, and the following data were obtained:
\begin{tabular}{ll}
\hline Time (s) & {$\left[\mathrm{CH}_{3} \mathrm{NCl}(M)\right.$} \\
\hline 0 & $0.0165$ \\
2,000 & $0.0110$ \\
5,000 & $0.00591$ \\
8,000 & $0.00314$ \\
12,000 & $0.00137$ \\
15,000 & $0.00074$ \\
\hline
\end{tabular}
(a) Calculate the average rate of reaction, in $M / \mathrm{s}$, for the time interval between each measurement. (b) Graph $\left[\mathrm{CH}_{3} \mathrm{NC}\right]$ versus time, and determine the instantaneous rates in $M / \mathrm{s}$ at $t=5000 \mathrm{~s}$ and $t=8000 \mathrm{~s}$.