C#: Create a new list from a given list of strings where each element is replaced by 4 copies of the string concatenated together

using System; // Importing the System namespace for basic functionality
using System.Collections.Generic; // Importing the namespace for working with collections
using System.Linq; // Importing the namespace for LINQ operations

namespace exercises // Defining a namespace called 'exercises'
{
    class Program // Defining a class named 'Program'
    {
        static void Main(string[] args) // The entry point of the program
        {
            // Creating a List of strings and assigning the result of the 'test' method
            List<string> mylist = test(new List<string>(new string[] { "1", "2", "3", "4" }));

            // Displaying the elements of 'mylist' in the console
            foreach (var i in mylist)
            {
                Console.Write(i.ToString() + " "); // Output each element followed by a space
            }
        }

        // Method that takes a List of strings and concatenates each string with itself four times
        public static List<string> test(List<string> nums_str)
        {
            // Using LINQ to concatenate each string in 'nums_str' with itself four times
            IEnumerable<string> s = nums_str.Select(x => x = x + x + x + x);

            // Converting the IEnumerable 's' to a new List and returning it
            return s.ToList();
        }
    }
}