不定项选择题 Which of the following fragments might cause errors?
   A．String s = "Gone with the wind"；
        String t = " good "；
        String k = s + t；
   B．String s = "Gone with the wind"；
        String t；
        t = s[3] + "one"；
   C．String s = "Gone with the wind"；
        String standard = s.toUpperCase()；
   D．String s = "home directory"；
        String t = s - "directory"；