In the various CPU scheduling algorithms we have studied thus far, when does preemption occur? Circle the best answer. (a) When a pre-defined timeslice expires (b) When a process changes its state to being blocked on I/O (c) When a higher-priority process arrives and wants to use the CPU (d) Both (a) and (b), but not (c) (e) Both (b) and (c), but not (a) (f) Both (a) and (c), but not (b)