It's 1-st point and two others.

thank you!
I choose 1st point and another point:
Part of my code:
//........
if     ( A*p[j].x + B*p[j].y >= C ) ++up;//if use A*p[j].x + B*p[j].y > C => WA13
else if( A*p[j].x + B*p[j].y < C ) ++down;
//........
if(up==down){
    printf( "1 %hd\n", ++i );
    return 0;
}