求助一道OG11verbal语法增补题

语法39

A study commisioned by the Department of Agriculture showed that if calves exercise and associated with other calves, they will require less medication and gain weight quicker than do those raised in confinement.

A. associated with other calves, they will require less medication and gain weight quicker than do

B. associated with other calves, they require less medication and gain weight quicker than

C. associate with other calves, they required less medication and will gain weight quicker than do

D. associate with other calves, they have required less medication and will gain weight more quickly than do

E. associate with other calves, they require less medication and gain weight more quickly than

选项E最后为什么没有do 啊？不懂~~

还有，为什么不用主将从现，全部都用现在时？

谢谢~~~

因为本题是两个动词的比较， 所以不需要加上do 那样就不确定do 到底指代的是

gain 还是 require 了

我明白了！！谢谢你~~