Set$-$up: a simple assembly line; $4$ workstations; $1$ operator per workstation

We now pay more attention to resource constraints.

Suppose there are only $3$ workers rather than $4.$ Analyze the process under the following conditions:

$(1)$

W orker W orkstation $1$A

$2$ B$,$C $3$D

$(2)$

W orker W orkstation $1$ A$,$ D $2$B $3$C

$(3)$

W orker W orkstation $1$ A$,$ C $2$B $3$D