Use the process flow diagram to determine which of these events has the greatest net benerit.

A$.$ reducing the flow time at Station A from $10$ to $8$ minutes.

B$.$ reducing the flow time at Station $D$ from $12$ to $10$ minutes.

C$.$ increasing the capacity at Station $C$ to $10$ units per hour.

D$.$ increasing the capacity at Station B to $12$ units per hour.