Which of the following fragments might cause errors？（）    

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"；