什么是局扇管理“七不让”？

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( )。

A．10,10

B．20,20

C．10,20

D．30,30