阅读下面程序: include<iostream.h> void funl(char a,char b) { char c; c=a;a=b;b=C; } void fun2(char &a,char b) { char c; c=a;a=b;b=c; } void fun3(char &a,char &b) { char c; c=a;a=b;b=C; } void main() { char a,b; a='A';b='B'; funl(a,b);cout<<a<<b; a='A';b='B'; fun2(a,b);cout<<a<<b; a='A';b='B'; fun3(a,b);cout<<a<<b; } 则该程序的输出为【 】。