单选题 执行如下程序后的输出结果是______.#include <iostream>using namespace std;class complex {private: 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; }};void disp(const complex &z) { cout << "(" << z.real( ) << ", " << z.image( ) << ")";}// ostream& operator<<(ostream &s, const complex &z) {// return s << "(" << z.real( ) << ", " << z.image( ) << ")";// }int main ( ) { complex x(1, -2), y(2, 3); disp(x+=y); // cout << (x+=y); return 0;}