Operating System (OS)
In MS-DOS, relocatable object files and load modules have extensions

None of these
.COM and .OBJ, respectively
.EXE and .OBJ, respectively
.OBJ and .COM or .EXE, respectively
.DAS and .EXE, respectively

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Operating System (OS)
Memory management is :

None of these
not used in modern operating system
not used on multiprogramming systems
replaced with virtual memory on current systems
critical for even the simplest operating systems

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