What is the output of this program?

    #include <iostream>
    using namespace std;
    void copy (int& a, int& b, int& c)
    {
        a *= 2;
        b *= 2;
        c *= 2;
    }
    int main ()
    {
        int x = 1, y = 3, z = 7;
        copy (x, y, z);
        cout << "x =" << x << ", y =" << y << ", z =" << z;
        return 0;
    }