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 question is from Official Guide. Therefore, because of copyrights, the complete question cannot be copied here. The question can be accessed at GMAT Club)
The sentence talks about what demographers need to do to develop more accurate population forecasts. Please note that the sentence uses “more” without ‘than’. Is it correct? Obviously, Yes. Why is it so obvious? Because it has been used in such manner in the non-underlined part. J
“more” can be used without ‘than’ when the second part of the comparison is clear. In this sentence, we are clear that the comparison is with the present i.e. ‘more accurate population forecasts than the present ones”.
What do the demographers need to do? They have to know much more than they know presently. Know about what? Know about the social and economic determinants of fertility.
The only problem that I find with this sentence is the incomplete structure in the comparison. The sentence must say “demographers have to know a great deal more than they know now”. As is, “now” after ‘than’ is not parallel to anything in the first part of the comparison.
(A) Incorrect. For the reason mentioned above.
(B) Incorrect. “economical” means inexpensive. Clearly, we’re not talking about inexpensive determinants of fertility!! We’re talking about the monetary determinant of fertility, and to communicate so, we have to use “economic”.
(C) Incorrect. Repeats the error of option B.
(D) Correct. The error in the original sentence has been corrected. The use of “would” is correct. “would” is used to talk about future events too. For example: “I would try to study”.
(E) Incorrect. Repeats the comparison error of the original sentence.