下面程序的运行结果是( )。
#included<iostream.h>
class base
protected:
int a;
public:
base()cout<<"0";
;
class base1:virtual base
public:
base1()cout<<"1";
;
class base2:virtual base
public:
base2()cout<<"2";
;
class derived:public base1,public base2
public:
derived()cout<<"3";
;
void main()
derived obj;
cout<<endl;
A.0123
B.3120
C.0312
D.3012