Could not load file or assembly 'System.Data.Linq, Version=4.0.0.0

The error is :

Could not load file or assembly 'System.Data.Linq, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089' or one of its dependencies. The system cannot find the file specified.

I have tried many solutions find on Google search and even here but without success.

I have tried with :

<add assembly="System.Data.Linq, Version=3.5.0.0, 
 Culture=neutral, PublicKeyToken=B77A5C561934E089"/>

But on the page I have error on this line because Linq not working :

string ns = string.Join(",", (from co in colorList select "?").ToList());

The .dll exists on the server:

My web.config file

<?xml version="1.0"?>
<configuration>
  <system.web>
    <customErrors mode="Off"/>
    <compilation>
      <assemblies>
        <add assembly="System.Data.Linq, Version=4.0.0.0, Culture=neutral, PublicKeyToken=B77A5C561934E089"/>
        <add assembly="System.Xml.Linq, Version=4.0.0.0, Culture=neutral, PublicKeyToken=B77A5C561934E089"/>
        <add assembly="System.Core, Version=4.0.0.0, Culture=neutral, PublicKeyToken=B77A5C561934E089"/>
        <add assembly="System.Data.Entity, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089"/>
      </assemblies>
    </compilation>
  </system.web>
</configuration>

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