Hello Everyone!
Let's tackle this question, one issue at a time, and come up with the correct choice! First, let's take a quick look at the original question, and highlight any major differences between the options in orange:
To develop more accurate population forecasts, demographers have to know a great deal more than now about the social and economic determinants of fertility.
(A) have to know a great deal more than now about the social and economic
(B) have to know a great deal more than they do now about the social and economical
(C) would have to know a great deal more than they do now about the social and economical
(D) would have to know a great deal more than they do now about the social and economic
(E) would have to know a great deal more than now about the social and economic
After a quick glance over the options, we see a few places we can focus on:
1. have to know / would have to know
2. now / they do now
3. economic / economical
Let's start with #3 on our list: economic vs. economical. This is an easy way to eliminate 2-3 options because it's a common mistake we see on the GMAT:
economic = dealing with the economy
economical = inexpensive, cheap, thrifty, etc.
Since the sentence is talking about the economy, we can eliminate options B & C because they use the word "economical," which isn't the correct word for this sentence. That was quick, right?
Let's move up to #2 on the list: now vs. they do now. Here are the remaining sentences with the non-underlined parts added in to better catch any problems:
(A) To develop more accurate population forecasts, demographers have to know a great deal more than now about the social and economic determinants of fertility.
This is INCORRECT because it creates a non-parallel comparison. We are supposed to compare what demographers would have to know (knowledge) to what demographers know now (knowledge). This is comparing what demographers would have to know (knowledge) to now (time), which isn't parallel!
(This is also wrong because "have to know" isn't how to write a conditional statement. "Would have to know" is a condition, which would make this a conditional statement.)
(D) To develop more accurate population forecasts, demographers would have to know a great deal more than they do now about the social and economic determinants of fertility.
This is CORRECT! It compares what demographers would have to know (knowledge) to what they know now (knowledge), which is parallel! It's also best to use "would have to know" to indicate this is a conditional statement, and it does that correctly!
(E) To develop more accurate population forecasts, demographers would have to know a great deal more than now about the social and economic determinants of fertility.
This is INCORRECT because it creates a non-parallel comparison. We are supposed to compare what demographers would have to know (knowledge) to what demographers know now (knowledge). This is comparing what demographers would have to know (knowledge) to now (time), which isn't parallel!
There you have it - option D is the correct choice! Even if you weren't confident on how to deal with conditional statements and verb tenses, you could still answer this question by focusing on parallel comparisons and proper word usage!
Don't study for the GMAT. Train for it.
