Merge two (or more) lists into one, in C# .NET

Is it possible to convert two or more lists into one single list, in .NET using C#?

For example,

public static List<Product> GetAllProducts(int categoryId){ .... }
.
.
.
var productCollection1 = GetAllProducts(CategoryId1);
var productCollection2 = GetAllProducts(CategoryId2);
var productCollection3 = GetAllProducts(CategoryId3);