If a diesel engine rotates slowly when cranked, but does not fire, the（）
A. fuel control rack had admitted excessive fuel
B. engine speed does not match the fuel rack setting
C. engine has failed to reach its firing speed
D. starter pinion and ring gear contact is not correct
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
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