What is output:

using System;

$,$ class MainClass $($

static void Main$($string$]$ args$)\{$

Senior senior $=$ new Senior$()$;

Student student $=$ senior;

student. study$()$;

$\}$

class Student

public Student $)$

Console.Write$("1")$;

$\}$

public void study$)\{$

Console.Write$("2")$;

$\}$

$\}$

class Senior: Student $\{$

public Senior$()\{$

Console.Write$("3")$;

$\}$

public void study$)\{$

Console.Write$("4")$;