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 operato-(point p1)
point p;
int px=x-p1.x;
int py=y-p1.y;
return point(px,py);
void print()
cout＜＜x＜＜","＜＜y＜＜endl;
;
int main()
point p1(10,10), p2(20,20);
p1=p1+p2;
p1.print();
return 0;
执行后的输出结果是( )a
A．10,10
B．20,20
C．10,20
D．30,30