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:
$S \rightarrow aB$ $S \rightarrow bA$
$B \rightarrow b$ $A \rightarrow a$
$B \rightarrow bS$ $A \rightarrow aS$
$B \rightarrow aBB$ $S \rightarrow bAA$
For the string $aabbab$, how many derivation trees are there?
- $1$
- $2$
- $3$
- $4$