سلام من تازه mvcرو شروع کردم و برنامه تمرینی کrun میکنم ارور زیر رو میده لطفا راهنماییم کنید

Server Error in '/' Application.

Compilation Error

Description: An error occurred during the compilation of a resource required to service this request. Please review the following specific error details and modify your source code appropriately.

Compiler Error Message: CS0234: The type or namespace name 'Ajax' does not exist in the namespace 'System.Web.Mvc' (are you missing an assembly reference?)

Source Error:

Line 22:     using System.Web.WebPages;
Line 23:     using System.Web.Mvc;
Line 24:     using System.Web.Mvc.Ajax;
Line 25:     using System.Web.Mvc.Html;
Line 26:     using System.Web.Routing;

Source File: c:\Users\Mohammad\AppData\Local\Temp\Temporary ASP.NET Files\root\a511cb00\4e031cea\App_Web_index.cshtml.a8d08dba.0b8cnz_u.0.cs    Line: 24