How do i respond to a river c/r

[Sightless]

no real reads, my 2nd orbit at the table or so.
MP Seems decent, didnt do anything out of the ordinary
His C/R at the river confused me, and I was really unsure how to respond to it.

Any advice?

Party Poker 1/2 Hold'em (6 max, 6 handed) FTR converter on zerodivide.cx

Preflop: Hero is CO with K[img]/images/graemlins/club.gif[/img], A[img]/images/graemlins/heart.gif[/img].
UTG calls, MP calls, <font color="#CC3333">Hero raises</font>, <font color="#666666">1 fold</font>, SB calls, <font color="#666666">1 fold</font>, UTG calls, MP calls.

Flop: (9 SB) 9[img]/images/graemlins/club.gif[/img], K[img]/images/graemlins/heart.gif[/img], T[img]/images/graemlins/club.gif[/img] <font color="#0000FF">(4 players)</font>
SB checks, UTG checks, <font color="#CC3333">MP bets</font>, <font color="#CC3333">Hero raises</font>, SB calls, UTG folds, MP calls.

Turn: (7.50 BB) K[img]/images/graemlins/spade.gif[/img] <font color="#0000FF">(3 players)</font>
SB checks, MP checks, <font color="#CC3333">Hero bets</font>, SB calls, MP calls.

River: (10.50 BB) 3[img]/images/graemlins/diamond.gif[/img] <font color="#0000FF">(3 players)</font>
SB checks, MP checks, <font color="#CC3333">Hero bets</font>, SB folds, <font color="#CC3333">MP raises</font>, <font color="green">HERO?? </font>