Respuesta :

The slope of every horizontal line is zero.  The slope is calculated by (y2 - y1) / (x2 - x1).  For every point on a horizontal line, its y-coordinates are the same. e.g. (2,8), (4,8), (6,8) etc.  Hence (y2 - y1) = 0 always.  The answer is (a) 0