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";
A.Ⅱ、Ⅲ
B.Ⅱ、Ⅳ
C.Ⅰ、Ⅳ
D.Ⅲ、Ⅳ