Sentence Correction

Hi Everyone,
I have learnt form my past OG experiences that 'would/could' should be used only when we talk about future from the view of past. Also MGMAT SC strategy guide says that whenever would/could is used another past tense is necessitated.

I faced a question from GMAT prep which contradicted the above view. Kindly explain whether the usage of 'would' is justified or it is just that D is the best among the bad options.

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

* have to know a great deal more than now about the social and economic
* have to know a great deal more than they do now about the social and economical
* would have to know a great deal more than they do now about the social and economical
* would have to know a great deal more than they do now about the social and economic
* would have to know a great deal more than now about the social and economical

Stacey and Ron,
Please let me know if this constitutes a copyright violation. If yes I will remove the post immediately. I tried posting in MGMAT forums but for some reason it did not allow me to post even though I had registered my strategy guide there.

thanks,
Vishnu