Question from the C++ - Fundamentals test

Replace "..." with the good line of code in order to display objects of type Complex.

class Complex {
public:
    Complex(double x, double y) : x(x), y(y) {}
    double getX() const {return x;}
    double getY() const {return y;}
private:
    double x;
    double y;
};

ostream& operator<<(ostream& sortie, Complex z) {
    ...
    return sortie;
}

int main() {
    Complex z(3, 4);
    cout << z << endl;
    return 0;
}