单项选择题
A.σ>l1l2+σl3+σl4 B.σl1+σl2+σl3 C.σl1+σl2 D.σl1+σl3+σl4
A.σl1+σl3+σl4 B.σl1+σl2+σl3 C.σl1+σl2 D.σl1+σl2+σl3+σl4
A.C25 B.C30 C.C40 D.C45