下列的哪个程序段可能导致错误

A．String s = "hello"；String t = "good"；String k = s + t;

B．String s = "hello"；String t;t = s[3] + "one"；

C．String s = " hello "；String standard = s.toUpperCase( )；

D．String s = "hello"；String t = s + "good"；