Hi,

I think Ron is doing a great job trying to help you understand every rule in play on the GMAT. That said, you need to learn to use a general framework to aid you in tough situations such as this one. In this case just ask yourself whether there is ambiguity caused by omitting "the prices were."



First off, let's check on what's being compared - times when the prices were different. There are several ways this sentence can be written:

A. The product's prices are higher this year than the product's prices were last year
B. The product's prices are higher this year than the prices were last year
C. The product's prices are higher this year than they were last year
D. The product's prices are higher this year than last year
E. The product's prices are higher this year than last

In each of the versions, there is no ambiguity as to what is being compared. A and B, however, are awkward because of how wordy they are. In C at least, the pronoun they is used to make the sentence more concise with no change in meaning from A and B.

With D, the omission of "they were" doesn't degrade the meaning of the sentence. Why is that the case? Well, when you examine the sentence and focus on the comparison signal "higher X than Y" you notice that we're comparing when the prices were different. In this case, there isn't anything in the sentence to confuse this comparison in D, so it is completely fine to omit "they were"

On a final note, examine the omission of "year" at the end of E. It's pretty clear that "last" could only modify year so omitting year doesn't result in a loss of meaning. Still I strongly prefer D :)
Thanks,
Chris

_________________
Chris Brusznicki
MGMAT Instructor
Chicago, IL

Hi ron !
I also really confused about it, since in your prior post you said both "have to" and "would have to" make sense. so could you reply the post and make it clear?
thanks in advance.

hi,
please don't just mention "your prior post". i have almost 10,000 posts on this forum, so that's not particularly helpful.
i.e., please QUOTE any "prior post" to which you might refer. thanks.

--

either "have to..." or "would have to..." (or "will have to...", which doesn't appear in the choices) could work here.

"have to ..."
--> no implication about whether this is likely; just a matter-of-fact statement about what they need.
"would have to..."
--> implies that this is an unlikely (or even impossible) scenario.
"will have to..."
--> implies that this will be at issue sometime in the future. doesn't imply anything about whether demographers will be able to achieve this kind of knowledge, but says that they'll need it (and so will fail in their efforts if they don't succeed in obtaining the knowledge).

if you change "economical" to "economic" in choice (b), you would have another viable option.

first of all, thanks for ron's great explanation again! but i will say sorry for you ron. in my post, because i didn't make my question clearly. i will promise next time i will examine what i say before i post! ACTUALLY MY QUESTION IS what prepgmat09 mention above "can we not think of "they know" as omitted in the original choice."
that is
To develop more accurate population forecasts, demographers have to know a great deal more than (they know) now about the social and economic determinants of fertility.

i will wait for your next great explanation! thanks