Operating System (OS)
Most of the microcomputer's operating systems like Apple DOS, MS DOS and PC DOS etc. are called disk operating systems because

they are partly in primary memory and partly on disk
None of these
they are available on magnetic tapes
they are initially stored on disk
they are memory resident

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Operating System (OS)
A critical region is

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

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