正常情况下左反推由哪个液压系统供压（）
A.A液压系统
B.B液压系统
C.备用液压系统
D.地面液压源
参考答案：A
开放性角色
More has been learned about the Moon than any other of the Earth''s neighbors in space because of the Apollo program, which enabled men to walk on the Moon and bring back hundreds of pounds of________.
A．rocks
B．rock
C．stone
D．stones
