单项选择题

下列的哪个程序段可能导致错误
A)String s = "hello";
  String t = "good";
  String k = s + t;
B)String s = "hello";
  String t;
  t = s[3] + "one";
C)String s = " hello ";
  String standard = s.toUpperCase( );
D)String s = "hello";
  String t = s + "good";