system.web.extension problem

THIS is the error i am getting when i am compiling my project  

Parser Error Message: Could not load file or assembly 'System.Web.Abstractions,

Version=2.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35' or one of its

dependencies. The system cannot find the file specified

Source File: C:\Documents and Settings\Administrator\My Documents\data\Web-page adversting

impression 2\web.config    Line: 25 

Assembly Load Trace: The following information can be helpful to determine why the 

assembly 'System.W开发者_如何学运维eb.Abstractions, Version=2.0.0.0, Culture=neutral, 

PublicKeyToken=31bf3856ad364e35' could not be loaded.

Double-check the System.Web.Abstractions DLL is set to "Copy Local = true" (right click on the properties for the reference).