要是程序执行后的输出结果为ABCD, 应在横线处添加的语句( )。
#include<iostream>
using namespace std;
class A
public:A() cout<<'A';
class B:______
public:B() cout<<'B';
;
class C:virtual public A
public:C() cout<<'C';
;
class D:public B,public C
public:D() cout<<'D';
;
void main()D obj;
A) public A
B) private A
C) protected A
D) virtual public A