using System;

using System.Linq;

public class Program

{

    public static void Main()

    {

        Console.WriteLine("Hello World");

        int[] a1 = { 1, 2, 3, 4, 5 };

        int[] a2 = { 3, 4, 5, 6, 7 }; 

        var result = a1.Intersect(a2);

        Console.WriteLine(string.Join(", ", result));

        Console.WriteLine(" ");

        new Student().Greet("Carlos").Greet("Norma");

        Console.WriteLine(" ");

        var original = "ABCDE";

        var quoted = original.Quote();

        Console.WriteLine(original);

        Console.WriteLine(quoted);

    }

}

​

internal class Student

{

    public Student Greet(string name) 

    {

        Console.WriteLine("Hello " + name);

        return this;

    }

}

​

public static class StringExtension

{

    public static string Quote(this string str) 

    {

        return "\"" + str + "\"";

    }

}