That's another way of using the vertical line test. Put a ruler perpendicular to the x axis and going through a point. If the ruler hits only one point, then then if all the points plotted do the same thing, then the points make a function.
C says the same thing. Only 1 element in the domain (x) can be associated with 1 y value (the range). If there is more than 1 y value, then you do not have a function.
