下面的哪些程序片断可能导致错误? A. String s = "Gone with the wind"; String t = " good "; ring 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";