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";