填空题 以下程序运行时，输出到屏幕的结果中第一行是 1，第二行是 2，最后一行是 3。
#include＜iostream.h＞
class A{
public:
A(char *s)
{
cout＜＜S＜＜endl;
}
A(){};
};
class B:public A
{
public:
B(char *s1,char *s2):A(s1)
{
cout＜＜s2＜＜endl:
}
};
class C:public A
{
public:
C(char *s1,char *s2):A(s1)
{
cout＜＜s2＜＜endl:
}
};
class D:public B,public C
{
public:
D(char *s1,char *s2,char *s3,char *s4):B(s1,s2),C(s1,s3)
{
cout＜＜s4＜＜endl:
}
};
void main(void)
{
D *ptr=new D("students","study","C++","hard");
delete ptr;
}