operating system - When is the use of semaphores needed? (with 4 cases included as examples)

Question

So I am asked to find from which of these four cases is a semaphore needed to avoid the creation of a deadlock. I am not sure I have grasped this idea yet, so some help would be appreciated!

1. Keyboard
2. Network Card (wifi)
3. A space in memory which can be accessed by many process for change in the data (e.g. shared memory)
4. Reading from a optical disk

If I were to guess, I wound probably go with number one only, because the keyboard has to be accessed by only one process at a time. However, I am not sure about any of these. Thanks :)

Answer 1

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