Let M = (K, Σ , Γ, Δ, s, F) be a pushdown automaton, where

K = {s,f}, F = {f}, Σ  = {a, b}, \(\tau\) = {a} and

Δ = {((s,a,ϵ), (s,a)), ((s,b,ϵ), (s,a)), ((s,a,a), (f,ϵ)), ((f,a,a),(f,ϵ)), ((f,b,a), (f,ϵ))}

Which one of the following strings is not a member of L (M)?