IEnumerable<IEnumerable<T>> to IEnumerable<T> using LINQ

Question

How to split an IEnumerable of IEnumerables to one flat IEnumerable using LINQ (or someway else)?

Answer 1

enumerable.SelectMany(x => x)

Answer 2

var result = from e in enumerables
             from v in e
             select v;

Answer 3

IEnumerable<IEnumerable<int>> number_collections = ...
IEnumerable<int> = number_collections.SelectMany(x => x);