单项选择题

下面的哪些程序段可能导致错误 ( )
Ⅰ: String s = "Gone with the wind";
String t = "good ";
String k = s + t;
Ⅱ: String s = "Gone with the wind";
String t;
t = s[3] + "one";
Ⅲ: String s = "Gone with the wind";
String standard = s.toUpperCase();
Ⅳ: String s = "home directory";
String t = s-"directory":

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