单项选择题

A.HO REQUI RED→HO REQUEST→HO REQUEST ACK→HO COMMAND
B.HO REQUEST→HO REQUEST ACK→HO REQUIRED→HO COMMAND
C.HO REQUEST→HO REQUI RED→HO COMMAND→HO REQUEST ACK
D.HO REQUI RED→HO COMMAND→HO REQUEST→HO REQUEST ACK