Line graphs are a <em>rate of change. </em>Even if it is straight like an asymptote on the y-axis or x-axis. Thus you can say that a and d<em> </em>are wrong by that definition.
c might be correct, but it's not the most correct. Plus, you'd need context to know such a thing, which we don't have here. That leaves b, and it is indeed the right answer, because line graphs are continuous (at least for the most part) and they show a change over time.
