Four threads, A through D, arrive in alphabetic order at a scheduling queue two milliseconds apart from each other. Estimated running times are 8, 6, 4, and 2 milliseconds, respectively. Their externally determined priorities are 3, 1, 4, and 2 respectively, 4 being the highest priority. For each of the following scheduling algorithms, determine the mean turnaround time and mean waiting time Assume thread switching is instantaneous. Preemptive Priority Scheduling Preemptive Shortest Job First In both cases, show your working in detail and include Gantt charts or other visualisations of thread execution