Operating System (OS)
The necessary conditions needed before deadlock can occur?

No Mutual Exclusion, Hold and wait, Preemption, Circular Wait
Mutual Exclusion, Hold and wait, Preemption, No Circular Wait
Mutual Exclusion, Hold and wait, No Preemption, Circular Wait
Mutual Exclusion, No Hold and wait, Preemption, Circular Wait

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Operating System (OS)
A critical region is

a program segment that has not been proved bug-free
a program segment that often causes unexpected system crashes
one which is enclosed by a pair of P and V operations on semaphores
none is the above
a program segment where shared resources are accessed

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