AQo with a scary flop.. who bets the turn?

[AlmightyJay]

CO is uber uber passive - 70% VPIP, 0% PFR, 0.1 AF. He checks/calls with anything. However, he is capable of making an occasional bizarre, out-of-the-blue bluff with absolutely nothing. He once called my PFR with J6o, check/called the flop and turn with nothing, then bet out on the river into two players.

BB is super loose and somewhat aggressive postflop.

The flop is pretty frightening, but their calls could mean anything - A-high, straight draws, single hearts, slowplayed flushes, two pair in the case of CO. BB's call means he might have a flush, or a draw to the flush, but if he isn't slowplaying a flush, then I have him beat. He would c/r with top pair or better.

So, who bets this turn?

Party Poker 1/2 Hold'em (9 max, 9 handed) converter

Preflop: Hero is UTG with A[img]/images/graemlins/diamond.gif[/img], Q[img]/images/graemlins/spade.gif[/img].
<font color="#CC3333">Hero raises</font>, <font color="#666666">4 folds</font>, CO calls, Button calls, <font color="#666666">1 fold</font>, BB calls.

Flop: (8.50 SB) Q[img]/images/graemlins/heart.gif[/img], K[img]/images/graemlins/heart.gif[/img], 7[img]/images/graemlins/heart.gif[/img] <font color="#0000FF">(4 players)</font>
BB checks, <font color="#CC3333">Hero bets</font>, CO calls, Button folds, BB calls.

Turn: (5.75 BB) 4[img]/images/graemlins/club.gif[/img] <font color="#0000FF">(3 players)</font>
BB checks, <font color="blue">Hero...</font>