Answer (E) rests upon the assumtion that motivation does not have a maximum. What if motivation functions like a bucket of water or the charge in your cell phone rather than the height of a kid or how well you read? 

I'm not saying it does, I point that out only to show that its still the de facto answer because its the only one that pertains to the argument. It seems to me the lesson here is that it's easier/ quicker to rule out an answer on grounds of relevancy than actually engage with the merit of it's logic.