A.σ B.ts C.tr D.ess
A.bS+c/(S+1) B.bS–c/(S+a) C.b/S+c/(S+a) D.b/S+c/(S-a)
A.ωn/(S2+2ξωnS+1)(0<ξ<1); B.ωn/(S2+2ξωnS+1)(ξ=1); C.T2/(T2S2+2ξTS+1)(0<ξ<1); D.1/[S(TS+1)]