单选题 若有以下程序：
#include ＜iostream＞
using namespace std;
class Base

public:
Base()

x=0;

int x;
;
class Derived1 : virtual public Base

public:
Derived1()

x=10;

；
class Derived2: virtual public Base

public:
Derived2()

x=20;

；
class Derived: public Derived1,protected Derived2;
int main()

Derived obj；
cout ＜＜obj.x＜＜endl;
return 0;

该程序运行后的输出结果是( )。
A．20 B．30
C．10 D．0