Let $M = (K, Σ, Г, Δ, s, F)$ be a pushdown automaton, where
$K = (s, f), F = \{f\}, \Sigma = \{a, b\}, Г = \{a\}$ and
$Δ = \{((s, a, \epsilon), (s, a)), ((s, b, \epsilon), (s, a)), (( s, a, a), (f, \epsilon)), ((f, a, a), (f, \epsilon)), ((f, b, a), (f, \epsilon))\}$.
Which one of the following strings is not a member of $L(M)$?
- $aaa$
- $aabab$
- $baaba$
- $bab$