Which statement is true about finding the midpoint between two points located on a horizontal or vertical line ?
A . When finding the midpoint between two points on a vertical line, keep the y-coordinate and find the average of the x-coordinate
B . When finding the midpoint between two points on vertical line, keep x-coordinate and find the average of the y-coordinates
C . When finding the midpoint between two points on a horizontal line, the x-coordinate is always 0 and the y-coordinates are averaged
D. When finding the midpoint between two points on a horizontal line , the y-coordinate is always zero and the x-coordinates are averaged