Given a B-Tree that contains the keys $k$ and $2k$, we know the height of the tree will be reduced if we delete the key $k$.
Prove or disprove: The height of the tree will also reduce if we remove $2k$.
Now the solution is that it's true, but the explanation is lacking.. When the height of a B-Tree is reduced it means that the root only has one element and both it's children have the minimum amount of keys, but what's the connection between keys $k$ and $2k$? Why can't they be on opposite sides of the level of the leaves and for example when we remove $k$ we get an underflow and when we remove $2k$ we don't? I feel like I'm missing an important property of B-Trees and I just can't move on from it..