if you want a method of a class not to override what will you do?

You might want to try getting hold of a copy of Jon Skeet's book "C# in Depth" or "C# in a Nutshell" from O'Reilly. Even if you just skim read one or both of them you should be able to retain enough information to know what it is you want more detail on ... I have a terrible memory, but because I read articles and books I often go "I'm sure there is a method for this and it's called something like ..."
virtual/sealed/protected however is fairly basic stuff, so I would advise reading up on things like that before your next interview

Thank you!