Happy to help. Let's first take a look at the stimulus.
We're told that four market research companies were tasked with making estimations of the future populations of three mid-sized cities. For two of the three cities, their populations were relatively stable and the estimations varied little between the companies. However, for the third city that was growing rapidly, the companies had greatly varying estimates.
The question asks, "Which one of the following, if true, would best help explain why estimates of the current population of the rapidly growing city varied more than did current population estimates for the two other cities?"
We're basically looking for an explanation as to why the estimates of the companies for the third (rapidly growing) city varied so much. Perhaps rapidly growing cities have unpredictable growth patterns and are hard to model. Whatever the answer is, it needs to be a strong reason that explains why the companies arrived at different estimations.
(A) says, "Population changes over time are more uniform from one district to another in the rapidly growing city than in the two other cities."
If (A) were true, then the paradox would actually be deepened because if the district population changes in the fast-growing city are more uniform, this doesn't provide an explanation for why the estimates are so different. We can get rid of (A).
(B) says, "The population of the rapidly growing city is increasing largely as a result of a high birth rate."
If (B) were true, then we still have no answer for why the estimates varied so much, so this does nothing to explain the paradox. We can get rid of this choice.
(C) says, "The population of the rapidly growing city has a lower average age than the populations of either of the two other cities."
If the fast-growing city has a lower age, this still provides no explanation for why the companies had such varying estimates for this city. (C) does nothing for us, so we can get rid of it.
(D) says, "All population estimates of the rapidly growing city were produced first by estimating the current populations of the city’s districts and then by adding those estimates."
If (D) were true, we still have no answer as to why the companies had such varying estimates. In fact, (D) actually deepens the paradox because we would expect the companies to have more similar estimates if they were conducting their estimates in roughly the same way. We can get rid of (D).
(E) says, "Whereas the companies used different methods for estimating the current population of the rapidly growing city, the companies used the same method for the two other cities."
(E) explains why the companies had such different estimates. If they used different methods for the rapidly growing city, then it's reasonable to expect that they had different results from each other. This answer makes the results make sense, so it's the correct answer choice.
Does this make sense? Let us know if you have any more questions!
