【多选题】
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"；
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