Five jobs are ready for processing at time zero through a workstation. The details of the processing time and the due date of

completion is provided in the following table.

Using the shortest processing time $($SPT$)$ rule, it can be concluded that the average flow time for these five jobs is

a$.$ more than $12$ but less than or equal to $14$ days.

b$.$ more than $14$ but less than or equal to $16$ days.

c$.$ more than $10$ but less than or equal to $12$ days.

d$.$ more than $8$ but less than or equal to $10$ days.