$3\mathrm{.}2.$ Consider $1|$ chains $|{\displaystyle \underset{?}{\overset{?}{}}}{w}_j{C}_j$ with the same set of jobs as in Exer$-$

cise $3\mathrm{.}1.($a$).$ The jobs are now subject to precedence constraints which take the

form of chains:

$12$

$345$

$67$

Find all optimal sequences.