Source: Official Guide for GMAT Review 2016 Sentence Correction; #61

1

To develop more accurate population forecasts, demographers

To develop more accurate population forecasts, demographers have to know a great deal more than now about the social and economic determinants of fertility.

2 Explanations

1

ZOE ZHUYAN

Hi Mike McGarry,
How are you ?

I know here is a video to explain of this question, unfortunately, YOUTUBE is not available in China, I am afraid I need your further writing explanation.

I am struggling with A, D and E.
I can understand that comparison should be logical, meaning
for this case, before I read choices, the comparison is either future time VS present time, or future knowledge VS present knowledge,

Here is my reasoning , please point out my faults. thanks
A) have to know a great deal more than now about the social and economic
first, I think "have to know "is sensible, because it presents the necessary to get the knowledge in order to improve future accuracy.
second, I review choice A as omit, -- have to know a great deal more than( they know ) now.
"they" refers to demographers, know is a verb that appears preceding.

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
first, IMO, "would have to know" presents demographers' necessary in the future, I think of it as available as well.
second, both D and E have a comparison that "demographers have to know " VS "they know/do" .. I am not sure how should I distinguish D and E, because I think that E omits "they do".

have a nice day
>_~

Feb 18, 2017 • Comment

Hi Zoe,

Happy to help :)

The easiest way to eliminate (A) and (E) is for illogical comparison. (A) and (E) both have "have to know a great deal more than now..." This compares a verb "to know" to a noun "now." It's illogical to say we can know more than "now."

We need to compare like verb phrases, so we need "have to *know* a great deal more than they *do* now."

Second, we need "would have to" because this sentence is discussing a hypothetical case, in which demographers know a great deal more about X than they do now.

ZOE ZHUYAN

Thanks so much for your explanation

have a nice day
>_~