单选题 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;
}
执行这个程序的输出结果是______。