Which of the following statement is correct?

#include<iostream.h> 
int x, y; 
class CuriousTabTest
{
    public:
    CuriousTabTest(int xx = 0, int yy = 0)
    {
        x = xx;
        y = yy;
        Display(); 
    } 
    void Display()
    {
        cout<< x << " " << y << " ";
    }
};
int main()
{
    CuriousTabTest objBT(10, 20); 
    int &rx = x; 
    int &ry = y; 
    ry = x;
    rx = y;
    cout<< rx--; 
    return 0; 
}

#include<iostream.h>
double CuriousTabFunction(double, double, double = 0, double = 0, double = 0);
int main()
{
    double d = 2.3;
    cout<< CuriousTabFunction(d, 7) << " ";
    cout<< CuriousTabFunction(d, 7, 6) << endl;
    return 0; 
}
double CuriousTabFunction(double x, double p, double q, double r, double s)
{
    return p +(q +(r + s * x)* x) * x;
}

#include<iostream.h> 
int main()
{
    int x = 10;
    int &y = x;
    x++;
    cout<< x << " " << y++;
    return 0; 
}

#include<iostream.h> 
class CuriousTabBase
{
    int x;
    public:
    CuriousTabBase(int xx = 0)
    {
        x = xx; 
    }
    void Display()
    {
        cout<< x ;
    }
};
class CuriousTabDerived : public CuriousTabBase
{
    int y; 
    public:
    CuriousTabDerived(int yy = 0)
    {
        y = yy;
    }
    void Display()
    {
        cout<< y ;
    }
};
int main()
{
    CuriousTabBase objBase(10); 
    CuriousTabBase &objRef = objBase;

    CuriousTabDerived objDev(20); 
    objRef = objDev;

    objDev.Display(); 
    return 0; 
}

#include<iostream.h> 
int main()
{
    int x = 10;
    int &y = x;
    x = 25;
    y = 50;
    cout<< x << " " << --y;
    return 0; 
}

#include<iostream.h> 
class CuriousTab
{
    int x, y; 
    public:
    void SetValue(int &xx, int &yy)
    {
        x =  xx++;
        y =  yy; 
        cout<< xx << " " << yy;
    }
};
int main()
{
    int x = 10;
    int &y = x;
    CuriousTab objCuriousTab;
    objCuriousTab.SetValue(x , y);
    return 0; 
}

#include<iostream.h> 
enum xyz
{
    a, b, c
};
int main() 
{
    int x = a, y = b, z = c;
    int &p = x, &q = y, &r = z;
    p = ++x;
    q = ++y;
    r = ++c;
    cout<< p << q << r;
    return 0;
}

#include<iostream.h> 
int main()
{
    int m = 2, n = 6;
    int &x = m;
    int &y = n;
    m = x++; 
    x = m++;
    n = y++;
    y = n++;
    cout<< m << " " << n; 
    return 0; 
}