单项选择题
A.σl4+σl5+σl6 B.σl3+σl4+σl5 C.σl5 D.σl4+σl5
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