QUESTION $1$

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

M$2$

M$3$

M$1$

None