Help understand $\text{handle}$ in parsing problem

Question

The BNF is defined as followed:

S -> aAb | bBA 
A -> ab | aAB
B -> bB | b

The sentence is:

aaAbBb

And this is the parse tree:

Phrases: aaAbBb, aAbB, bB
Simple Phrases: bB
Handle: ?

From the book, handle is defined as followed: B is the handle of the right sentential from y = aBw if and only if:
$S ->_{rm} \cdot aAw ->_{rm} aBw$

So in my case, what's the handle? Any idea?

Thanks,
Chan