PokerStars No-Limit Hold'em, $0.50 BB (6 handed) Hand History Converter Tool from FlopTurnRiver.com (Format: FlopTurnRiver)

MP ($20.80)
CO ($26.40)
Button ($222.60)
SB ($85.70)
Hero ($50)
UTG ($35.60)

Preflop: Hero is BB with J, 9.
2 folds, CO calls $0.50, Button calls $0.50, 1 fold, Hero checks.

Flop: ($1.75) 9, J, 3 (3 players)
Hero bets $1.5, CO calls $1.50, Button folds.

Turn: ($4.75) 8 (2 players)
Hero bets $4, CO calls $4.

River: ($12.75) K (2 players)
Hero ???

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Villian is an unknown.

What's your river play here?