单选题
有如下程序：
#include
using namespace std;
class Base{
int m;
public:
Base(){ cout<<"B1"; }
Base(int m1): m(m1){ cout<<"B2"; }
};
class Mid1: virtual public Base{ };
class Mid2: virtual public Base{ };
class Derived: public Mid2, public Mid1{
public:
Derived(int n): Base(n){ cout<<"D"; }
};
int main(){
Derived obj(5);
return 0;
}
运行上述程序的输出结果是