这个题选哪个答案，为什么？
下面的哪一个程序片断可能导致错误。 (   )
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";
  s = s + "directory";

这个题选哪个答案，为什么？
下面的哪一个程序片断可能导致错误。 (   )
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";
  s = s + "directory";