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Level B-21 is the fifth course of the Bonus 3 set and the twenty-first Bonus Level overall in Mario vs. Donkey Kong: Tipping Stars. It is set in Clattering Cabin and is unlocked by earning 84 trophies.
The course is composed of a large number of chambers and halls, separating the Minis. Series of Blue Lifts and two Split Pipes are connecting some of these areas. Throughout the course, Minis depend on each other to press Color Switches that disable Color Blocks, or pass Hammers that are used to destroy Rocks blocking the way. After grabbing a pair of Hammers, a Mini can then slide on a slope or go down a Blue Lift to throw them upward, potentially entering another room where they can be taken by another toy that requires the Hammers to advance. Eventually, all Minis converge into a room on the bottom right, from where they can access the Goal Door via a Blue Lift, provided they destroy the heap of Rocks blocking it.
The M Coin can be found in a small room under the chamber where the Mini Luigi in the middle is found. The room can be entered with a Blue Lift, but beyond the coin lies a wall of spikes. However, under the coin is an escape aperture blocked by a cluster of Rocks. One of the other toys should take hold of a pair of Hammers and use them to crush these Rocks. With the path now free, the Mini Luigi from earlier can descend with a Blue Lift to the coin and grab it, falling through the aperture before touching the spikes ahead.
The following data applies in both the Nintendo 3DS and the Wii U versions of the game.