Consider a process with three machines in series: M$1$ with a processing $($cycle$)$ time of $5$ min$/$unit$,$ M$2$ with a processing time of $10$ min$/$unit$,$ and M$3$ with a processing time of $7$ min$/$unit$.$ A product goes through M$1$ first, then M$2,$ and finally M$3.$The bottleneck is: