You create a Microsoft ASP.NET application by using the Microsoft .NET Framework version 3.5. When you access the application in a Web browser， you receive the following error message： "Service Unavailable".  You need to access the application successfully.  What should you do？（）

• A、Start Microsoft IIS 6.0. 
• B、Start the Application pool. 
• C、Set the .NET Framework version. 
• D、Add the Web.config file for the application.