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