Consider a process with three machines in series: M$1$ with a processing $($cycle$)$ time of $5mi\frac{n}{?}$ unit, M$2$ with a processing time of $10mi\frac{n}{u}$nit, and $M3$ with a processing time of $7mi\frac{n}{?}$ unit. A product goes through $M1$first, then $M2,$ and finally $M3.$ The bottleneck is

M$2$

M$3$

None

M$1$