X represents one component, for which M$1$ and M$2$ need to process sequentially. Y represents another component to be added to X so as to make one final product; M$3$ and M$4$ work independently and each can process a component Y by itself. The last step $($step $4)$ requires an assembly operation, where M$5$ will take a component X and a component Y from the buffer $($represented by the triangle$)$ and assemble them into a final product. The bottleneck of the process is Question $37$ options: Step $1$ Step $2$ Step $3$ Step $4$