单选题 有如下程序： #include ＜iostream＞ using namespace std; class point { int x, y; public: point( int vx, int vy ) { x = vx; y = vy; } point ( ) { x = 0; y = 0; } point operator+( point pl ) { point p; int px = x + p1.x; int py = y + p1.y; return point( px, py ); } point operator-( point p1 ) { point p; int px = x - p1.x; int py = y - p1.y; return point( px, py ); } void print() { cout＜＜x＜＜", "＜＜y＜＜end1; } }; int main () { point pl ( 10, 10 ), p2 ( 20, 20 ); p1 = p1 + p2; p1.print (); return 0; } 执行后的输出结果是( )。