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Mario Golf
Golf Left-Handed:
Press and hold the SELECT button. While holding this, press the A button. If you choose Mario, Wario, Luigi, or Club Champs, that golfer will be a left-handed player.

Infinite Retries:
Before completing a hole, save your game and exit - don't turn off the Gameboy. Once you return to your game, you will be able to replay the hole again.