Sentence Correction

Q32:
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 economical

This is a simple condition case....lets restate in his way
If you have to develop......now if we look at the options it is clear that B fits the best.

Jangojess is right about this being conditional. That means that you can eliminate all the answer choices not in the conditional form -- A and B are out.

The next part is would have to know a great deal more than they do now. You have to have that part to avoid ambiguity. So E is out.

The difference between D and C is economic vs. economical. There is no real dichotomy between these two words that I know of -- they're both adjectives. But 'economic' is the word used to modify terms like trends, potential, market, etc, and 'determinant' is a word like this. You don't say "economical trends" and you don't say "economical determinant". Therefore it's D.