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 p1 )

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 p1( 10, 10 ), p2( 20, 20 );
p1 = p1 + p2;
p1.print();
return ();执行后的输出结果是( )。

A．10, 10

B．20, 20

C．10, 20

D．30, 30