单选题 若有以下程序：
#include ＜iostream＞
using namespace std;
class Base
{
private:
int a,b;
public:
Base(int x, int y)
{
a=x;
b=y;
}
void show()
{
cout＜＜a＜＜", "＜＜b＜＜end1;
}
};
class Derive : public Base
{
private:
int c, d;
public:
Derive(int x, int y, int z,int m):Base(x,y)
{
c=z;
d=m;
}
void show()
{
cout＜＜c＜＜", "＜＜d＜＜end1;
}
};
int main ( )
{
Base b(50,50) ,*pb;
Derive d(10,20,30,40);
pb=&d;
pb-＞show {);
return 0;
}