So I pretty much always narrow down a question that asks about the relationship of two variables to the graphs that show the correct correlation except for one is asymptotic and one is linear... However I always seem to pick the wrong one. Whats the general rule from predicting the graph from a formula. Take for example this problem in Physics Book 2 section 8 on electrostatics and electormagnetism:
Why is it asymptotic and NOT linear...? Any help is greatly appreciated.

Why is it asymptotic and NOT linear...? Any help is greatly appreciated.