中断扫描机构是( )扫描一次中断寄存器。

A．每隔一个时间片
B．每条指令执行周期内最后时刻
C．每当进程释放CPU
D．每产生一次中断

why do some stations keep silence?（）

A.Because they are not engaged in the distress traffic

B.they will affect the transmission of the distress traffic

C.The powers of these stations are very weak

D.They are not in charge of search and rescue