Puzzles from Pentacubes

Three construction puzzles with three answers. The answers, "keys", are made from assemblies of 20 cubes. Consider the keys to be a corner of a walled garden where a flower pot can take 3 different positions. The puzzle pieces are sets of four 5-cubes, ("pentacubes"). Puzzle 10, the trivial one, has obvious answers. Puzzle 9, the easy one, is clued by Puzzle 10. Puzzle 31 is difficult. OpenSCAD code is included.