Which of the valves listed is NOT considered to be a hydraulic system directional control valve?
A.Spring-centered valve
B.Unloading valve
C.Three-position valve
D.Two-position valve

A.1mL含（

A.中心静脉压低，血压低
B.中心静脉压低，血压正常
C.中心静脉压高，血压低
D.中心静脉压高，血压正常
E.中心静脉压正常，血压低