[单选题]A.B.C.D.
[单选题]A.B.C.D.E.
[单选题]A.-1B.-2C.1D.3
[单选题]A.-1.2kN/㎡B.9.8kN/㎡C.1.2kN/㎡D.-9.8kN/㎡
[单选题]A.NA>NBB.NA<NBC.NA=NB=0D.NA=NB≠0