A plant have five parts $($P$1,$ P$2,$ P$3,$ P$4,$ and P$5)$ are to be processes on five machines $($M$1,$ M$2,$ M$3,$ M$4,$ and M$5),$ accordingly. The detailed processes are given below:

P$1$s processes sequence is M$3,$ M$4,$ and M$5.$

P$2$s processes sequence is M$3,$ and M$4.$

P$3$s processes sequence is M$1,$ M$2,$ and M$4.$

P$4$s processes sequence is M$1,$ M$3,$ M$4,$ and M$5.$

P$5$s processes sequence is M$1$ and M$2.$

Now the plant would apply group technology to manufacture these parts. By Rank Order Clustering $($ROC$)$ Algorithm, a series diagonal blocks $($left upper and right down$)$ will be formed. The first left upper block will be named Cell $1,$ and next Cell $2,$ and so on so for. Then, which machine is bottleneck machine? $($Bottleneck machine is the machine who not only work in its own cell, but also need to process the parts from other cells.$)$