NL100: odd spot with set of kings on monotone flop

[Some9]

I am still a beginner trying to learn the game.
I felt kind of lost in this spot, comments appreciated.


NL$100 Real Money 6max

<font color="#666666">effective stacks
MP: $40
Hero: $120
BTN: $202
BB: $98</font>

Hero is CO with K [img]/images/graemlins/heart.gif[/img] K [img]/images/graemlins/club.gif[/img]

1 fold
MP calls $1 He was typical loose passive player
1 fold
Hero raises to $5
BTN calls $5 He appeared to be a decent TAG
1fold
BB calls $4 Seemed weak tight
MP calls $4

Flop ($20, 4 players) A[img]/images/graemlins/diamond.gif[/img] K[img]/images/graemlins/diamond.gif[/img] 9[img]/images/graemlins/diamond.gif[/img]
BB checks
MP bets $1
Hero [img]/images/graemlins/confused.gif[/img]

So how do we continue from here?