Why does Bitcoin send the "change" to a different address? For example I have an address (A) with 20 BTC and send 9 BTC to address (B).  Bitcoin will create a "change" portion of the transaction where the remaining 11 BTC goes to address (C).

Why not send the change back to A?

Instead of:


Initial balance: 
A: 20 BTC 
B: 0 BTC 
C: 0 BTC

Transaction Pair: 
A -&gt; B 9 BTC 
A -&gt; C 11 BTC

End result 
A: 0 BTC 
B: 9 BTC 
C: 11 BTC



Why not?


Initial balance:
A: 20 BTC
B: 0 BTC

Transaction Pair:
A -&gt; B 9 BTC
A -&gt; A 11 BTC

End result
A: 11 BTC
B: 9 BTC
Two reasons:


So whoever sent to your "A" address can't claim to have sent you 31 bitcoins ("See!  Look on block explorer, there are two transactions sending to "A", one for 20 bitcoins and one for 11-- send me back the extra 11!")
Using a new change address makes it more difficult for other people to track of how many bitcoins you have or where you're spending them.