Is FileMode necessary in C# FileStream?

last week I had interview for intern from C#. My interviewer asked me what wrong is with this code:

public void test(){
        FileStream fs = new FileStream(".//b", FileMode.CreateNew);
        fs.WriteByte(2);
        fs.Close();
    }

I haven't answered, but he gave me one week more to practice better my knowledge from C#. I thought maybe something with FileMode, but I'm not sure if there is for real something wrong.