多项选择题
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"