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";