Consider the CFG with $\left\{S, A, B\right\}$ as the non-terminal alphabet, $\{a, b\}$ as the terminal alphabet, $S$ as the start symbol and the following set of production rules:
$S \rightarrow aB$ $S \rightarrow bA$
$B \rightarrow b$ $A \rightarrow a$
$B \rightarrow bS$ $A \rightarrow aS$
$B \rightarrow aBB$ $S \rightarrow bAA$
Which of the following strings is generated by the grammar?
- $aaaabb$
- $aabbbb$
- $aabbab$
- $abbbba$