单选题：class X{

class X{
int a;
public:
X(int x=0) {a=x;}
};
class Y:public X {
int b;
public:
Y(int x=0,int y=0):X(y)) { b=x;}
};
下述语句存在错误的是（ ）@[D](2)

A. X *pa=new Y(1,2);
B. X a1=Y(1,3);
C. Y b1(2,3); X & a3=b1;
D. X a4(10); Y b2=a4;




A.X *pa=new Y(1,2);
B.X a1=Y(1,3);
C.Y b1(2,3); X & a3=b1;
D.X a4(10); Y b2=a4;


答案：D