combine two arraylist c#

List<string> a = new List<string>();
List<string> b = new List<string>();

a.AddRange(b);

class Program

    {
        static void Main(string[] args)
        {
            ArrayList CountryList1 = new ArrayList();
            ArrayList CountryList2 = new ArrayList();

            CountryList1.Add("Pakistan");
            CountryList1.Add("Nepal");
            CountryList2.Add("Butan");
            CountryList2.Add("Srilanka");
          
            CountryList1.AddRange(CountryList2);
        }

    }