单项选择题

有如下程序:
#include <iostream.h>
class x
protected:
int a;
public:
x()

a=1;

;
class x1 : virtual public x
public:
x1()

a+=1;
cout<<a;

;
class x2 : virtual public x
public:
x2()
a+=2;
cout<<a;
;
class y : public xl,public x2
public:
y()
cout<<a<<end1;
;
int main()
y obj;
return O;
该程序运行后的输出结果是( )。

A.1
B.123
C.242
D.244