关于比较，OG16三道自相矛盾，左右互搏的题目，求大神回答，相信很多同学都碰到过

聪明蛋

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
economic 是"经济上的"，economical是"节约的";
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
D

A large rise in the number of housing starts in the coming year should boost new construction dollars by several billion dollars, making the construction industry's economic health much more robust than five years ago.
(A) making the construction industry's economic health much more robust than five years ago
(B) and make the construction industry's economic health much more robust than five years ago
(C) making the construction industry's economic health much more robust than it was five years ago
(D) to make the construction industry's economic health much more robust than five years ago
(E) in making the construction industry's economic health much more robust than it as five years ago
选C
这两个就都是要补足，不省略的情况了，不懂究竟什么时候补足什么时候省略