b) Multi-tasking :

• * Because lots of programs need to be run at once (especially in a Windows environment), operating system need to be able to multi-task.
  
• * Multi-tasking makes it appear as if multiple programs are running simultaneously, but actually that’s just an illusion caused by the fast processing speed of the CPU.
  
• * What actually happens is that the CPU divides its time between various tasks – it performs part of the task on one application, followed by part of a task on another application. It then returns to the first application, and so on..