问大家几道题

In a certain cache system, the modified cache block is written to main memory only when the block is replaced. All of the  following are used to describe this policy except__.
A  store-in               B  write-back       C  copy-back      D write-through

Assume that an operating system uses a round-robin scheduler. The process's quantum is 20msec, and the context         switch  is 5 msec. What percentage of the CPU's time is spent on administrative overhead?__.
A  5%                     B 15%                    C 20%              D25%


Which MS-DOS command is correct?__
A     DEL /S*BAT
B     DEL *.EXE/SP
C    DEL *.OBJ>NUL:
D    DEL *.com*.sys