下面的哪些程序段可能导致错误? ( ) Ⅰ：String s='Gone with the wind'; String t='good'; String k=s+t; Ⅱ：String s='Gone with the wind'; String t; t=s[3]+'one'; Ⅲ：String s='Gone with the wind'; String standard=s.toUpperCase(); Ⅳ：String s='home directory'; String t=s-'directory';A.Ⅱ、ⅢB.Ⅱ、ⅣC.Ⅰ、ⅣD.Ⅲ、Ⅳ
A.toUpperCase();
B.Ⅱ、Ⅲ
B.Ⅱ、Ⅳ
C.Ⅰ、Ⅳ