单选题
#include
class Base1
{
public:
Base1() {cout<<"B1";}
};
class Base2:public Base1
{
public:
Base2() {cout<<"B2";}
};
class Derived:public Base2
{
public:
Derived (){cout<<"D";}
};
int main()
{Derived d;
return 0;}
执行上面的程序将输出（ ）
A、DB1B2 B、DB2B1 C、B2B1D D、B1B2D