C#でArrayListに項目を追加するにはどうすればよいですか?

ArrayList arr= new ArrayList();

ArrayList arr1 = new ArrayList();
arr1.Add(30);
arr1.Add(70);

using System;
using System.Collections;

public class MyClass {
   public static void Main() {
      ArrayList arr1 = new ArrayList();
      arr1.Add(30);
      arr1.Add(70);

      ArrayList arr2 = new ArrayList();
      arr2.Add(200);
      arr2.Add(240);

      arr1.AddRange(arr2);
      for (int i = 0; i < arr1.Count; i++)
      Console.WriteLine(arr1[i]);
   }
}