Asp.net Build error?

when i run the my asp.net application it is showing this message can u help me.

Could not load file or assembly 'System.Web.Extensions.Design, Version=1.0.61025.开发者_运维知识库0, Culture=neutral, PublicKeyToken=31bf3856ad364e35' or one of its dependencies. The system cannot find the file specified.

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You need to check whether referenced assembly (System.Web.Extensions.Design) exists in GAC or Bin folder of your app.