Consider a problem of assigning four clerks to four tasks. The time $($hours$)$ required to complete the tasks are given below:

$\backslash$table$[[,,$TASKS,,$],[$Clerks$,$A$,$B$,$C$,$D$],[1,4,7,5,6],[2,-,8,7,4],[3,3,-,5,3],[4,6,6,4,2]]$

Clerk $2$ cannot be assigned to task A and clerk $3$ cannot be assigned to task $3.$ Find all the optimum assignment schedules.