Concept:A zero gradient means the line has no vertical change.Explanation:The gradient is given by m=x2−x1y2−y1.For m=0, we need y2−y1=0, so y2=y1.Hence, all points on the line have the same y-coordinate.A line with constant y-coordinate is parallel to the x-axis.Therefore, the line is horizontal.Answer:B. is horizontal.