My rephrasing of the argument:
Markings help drivers see better, but why isn't this the case in Greatwater?
If you try to come up with an answer to that, you would get something like..
(1) Maybe something is different about Greatwater conditions
(2) Some other factor must contribute to increase in the rate.
Look at answer choices:
A. Irrelevant because even if Greatwater has a lot of narrow roads, this does not explain why markings don't help in Greatwater
B. Irrelevant. This one doesn't explain either. If it were to say that Greatwater drivers consistently experience bad weather, then this becomes a valid choice. However, the choice doesn't say that bad weather is in Greatwater. This is just a general statement.
C. Irrelevant because we care about the resulting increase. What happened before has no relation with the increase.
D. Irrelevant. Does not explain why the markings doesn't help.
E. Bingo! Maybe it is because a lot of people use those road more. Since people use more road, and markings only help, not eliminate, the accidents, then this can be a factor. This is similar to the answer in (2).