A.
B.
C.
D.
[单选题]A.1800B.1600C.1500D.750
[单选题]A.B.C.D.
[单选题]A.F只能沿斜面向上B.F只能沿斜面向下C.F既可能沿斜面向上,也可能向下D.F=O
[单选题]A.2500mB.3000mC.3500mD.4000m