下面哪段语法执行正确()
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";