多选题：Java语言中，下面哪些代码片段会产生编译错误？

Java语言中，下面哪些代码片段会产生编译错误？



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

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


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


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




答案：B D