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

链霉素皮试的剂量为每0.1mL含（ ）A、50UB、150UC、250UD、2500UE、3000U

A.1mL含（

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血容量严重不足表现为

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

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