A.必要与充分 B.必要 C.充分 D.几何
A.平行于 B.相交与 C.垂至于 D.指向
A.mx(F)=0,my(F)=Fa,mz(F)=0 B.mx(F)=0,my(F)=0,mz(F)=0 C.mx(F)=Fa,my(F)=0,mz(F)=0 D.mx(F)=Fa,my(F)=Fa,mz(F)=Fa
如下: