using System;
using System.Collections.Generic;
using System.Linq;
public class Test
{
    public static void Main()
    {
        List<string> a = new[] { "a", "b", "c" }.ToList();
        List<string> b = new[] { "d", "e", "f" }.ToList();
        List<string> c = new[] { "g", "h", "i" }.ToList();
        List<string> d = new[] { "j", "k", "l" }.ToList();
        List<string> e = a.Union(b).Union(c).Union(d).ToList();
        Console.WriteLine(string.Join(", ", e));
    }
}