Created Unassigned: Compile error "Could not load file or assembly 'OpenXml' or one of its dependencies. The located assembly's manifest definition does not match the assembly reference. (Exception from HRESULT: 0x80131040) " [9379]

I dowmload OpenXml and ClosedXML for .Net 4.0 and refered to my web application. When I compile my application and I got the following error. What I missed?

Could not load file or assembly 'OpenXml' or one of its dependencies. The located assembly's manifest definition does not match the assembly reference. (Exception from HRESULT: 0x80131040)