多选题：下面的哪些Java程序片断可能导致错误。

下面的哪些Java程序片断可能导致错误。



A.java
String s = "Gone with the wind";
String t = " good ";
String k = s + t;


B.java
String s = "Gone with the wind";
String t;
t = s[3] + "one";


C.java
String s = "Gone with the wind";
String standard = s.toUpperCase();

D.java
String s = "home directory";
String t = s - "directory";



答案：B D