In the context-free grammar below, $S$ is the start symbol, $a$ and $b$ are terminals, and $\epsilon$ denotes the empty string
$S \rightarrow aSa \mid bSb \mid a \mid b \mid \epsilon$
Which of the following strings is NOT generated by the grammar?
- $aaaa$
- $baba$
- $abba$
- $babaaabab$