High speed circuit is often used in hydraulic machine tools to ().
A. increase the speed of working feed and improve the working efficiency of the system
B. increase the speed of the working mechanism when there is no load, and improve the working efficiency of the system
C. increase the machining speed of working mechanism and improve the working efficiency of the system
D. increase the oil pressure of the system to speed up the speed of working mechanism, and improve the working efficiency of the system