The question says , point varies in a direction , now when any point will vary in a direction , it will make a line .
If it varies in a direction with some angle , or we can say that the direction is also varying then it makes a curve .
If the line varies in two directions at the same time , it makes a surface ,
And
if the line varies in all directions then it makes a plane , and then it will have three coordinates.
Now here the point varies only in 1 direction . Hence it makes a line.
