题目内容：
有如下程序：#include<iostream>
using namespace std;
class Base1 {
public:
Base1(int d) {cout<<d;}
~Base1(){}
};
class Base2{
public:
Base2(int d) {cout<<d;}
~Base2(){}
};
class Derived:public Base1,Base2{
public:
Derived(int a,int b,int c,int d);
Base1(b),Base2(a),b1(d),b2(c) {}
private:
int b1;
int b2;
};
int main(){
Derived d(1,2,3,4);
return 0;
}
运行时的输出结果是（ ）。 A．1234
B．2134
C．12
D．21
参考答案：
答案解析：