题目选项分析

“In parallel constructions, if the verb in the second part of the construction is "absolutely the same" as in the first part of the construction, then that verb may be omitted in the secod part."
unfortunately, that principle does not apply here, since the verb that you're trying to omit is not exactly the same as its counterpart.
specifically, the first verb is in a hypothetical/conditional tense (would have to know...), while the second verb is in the present tense (they do [= know] now). you are not allowed to imply such tense transitions.
if you omit the verb, the omitted verb will be assumed to be in the same tense as its parallel counterpart. since you have to change tenses in this problem, you must include both verbs.

1. would have to 将来，和现在做比较，now
2. more than now容易产生歧义，补足they do 更好

than前后比较对象不一致

A选项 demographers know a great deal 与now相比，错误
D选项，know a great deal 与 they know now相比,比较成分更明确。

than 比较对象左边没有和“now” 对应的成分

ecnomic 经济学的 adj.
ecnomical 经济的；节约的；合算的 adj.
more than 表对比，与 they do 也就是 "现在" 作比较的话，就用 would have to

1. 如果是省略补充完整的话应该
demographers have to know a great deal more than they have to know now about the social and economic determinants of fertility.
前后的时态变化应该补充出主语与动作

now不平行

比较双方demographers，比较对象know，所以后面用now不正确

A的比较对象不平行。