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"；