java中值传递、引用传递及String类特性

指出下列程序运行的结果
public class Example{
String str=new String(“good”);
char[]ch={‘a’,’b’,’c’};
public static void main(String args[]){
Example ex=new Example();
ex.change(ex.str,ex.ch);
System.out.print(ex.str+”and”);
System.out.print(ex.ch);
}
public void change(String str,char ch[]){
str=”test okj”;
ch[0]=’g’;
}
}
A. good and abc
B. good and gbc
C. test ok and abc
D. test ok and gbc
为何选B？？？