World 6-B (New Super Mario Bros. 2)
From the Super Mario Wiki
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World 6-B is the second special level in World 6 of New Super Mario Bros. 2. The player needs five Star Coins to unlock this level. Once the player completes this level, it unlocks World 6- and skip World 6-3.
The player starts off on a safe platform, and must get across three platforms ahead. Throughout the first half of the level, three Flame Chomps appear at different locations. Once the player makes it to the end of the room, a pipe takes them to the next room. Alternatively, the player could find a hidden vine that leads to the second Star Coin, as well as the next room. In the second half of the level, the player encounters several Bullet Bills and Banzai Bills. Below are two Gold Rings which turn the Bullet Bills and Banzai Bills to gold, and above is a Red Ring and a Star Coin. There are also Flying ? Blocks in some areas. When the player reaches the end of the level, the last platform flashes and disappears, after which the Goal Pole appears.