using System;

using System.Text;

using System.Collections.Generic;

using System.Linq;

​

​

public class Program

{

    public static void Main()

    {

        var lst = new List<string>(){"A", "B", "C"};

        StringBuilder sb = new StringBuilder();

        lst.Select(str => sb.Append(str.ToLower()));

        Console.Write("Output: {0}", sb.ToString());

    }

}