Consider the following four processes with the arrival time and length of CPU burst given in milliseconds:
$\begin{array}{ccc} \textbf{Process} & \textbf{Arrival Time} & \textbf{Burst Time} \\ P_1 & 0 & 8 \\ P_2 & 1 & 4 \\ P_3 & 2 & 9 \\ P_4 & 3 & 5 \end{array}$
The average waiting time for preemptive SJF scheduling algorithm is ____________
- $6.5$ ms
- $7.5$ ms
- $6.75$ ms
- $7.75$ ms