单选题 有如下程序：
#include＜iostream＞
using namespace std;
class Complex
{
double re, im;
public:
Complex(double r, double i):re(r), im
(i){}
double real() const{return re;}
double image() const{return im;}
Complex& operator+=(Complex a)
{
re+=a.re;
im+=a.im;
return * this;
}
};
ostream &operator＜＜(ostream& s, const
Complex& z)
{
return s＜＜"("＜＜z.real()＜＜", "＜＜z.image()＜＜")";
}
int main()
{
Complex x(1, -2), y(2, 3);
cout＜＜(x+=y)＜
return 0;
}
执行这个程序的输出结果是______。