看下这个句子的哪种说法对。。

Agriculture showed that if claves exercise and associated with other claves, they require less medication and gain weight more quickly than do those raised in confinement.

这里需不需要加do 呢，正确答案里是没有的，可我觉得应该有。这个结构式属于主语比较的，A do sth than B do....显然是要加的啊，为什么答案没有
谢过先

A study commissioned 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

这道题看似考察平行结构， 实际是时态问题

主句用一般过去时 'showed'
宾语从句是‘条件状语从句+主句’ 的结构
状语从句：动词exercise 使用一般现在时，因为这属于科学发现，客观真理的范畴。所以associated应该也使用一般现在时。
排除A B两项

C 项中 require是过去时，
D项require是现在完成时。
但是C D两项中 ‘gain’ 使用的都是将来时，而require 和gain两个谓语动词又是并列的关系。矛盾
排除DC两项

只能选D了

的确是这道题，按照排除法是只能选E，没错
但是分析正确选项，就是理解不了为什么不加do。
然后把大全上比较的句子都翻遍了，貌似就这一题比较例外
有N分析如下：
gemj:这个句子之所以不补出，而且THAN的前面虽然有宾语，但是双重动宾结构。从语法上、逻辑上都不会产生those raised in confinement与weight或medication相比较的嫌疑。当然，就这个例子来说，如果补出，并不算多余。这才是补与不补均可的情况吧。

我觉得其实加不加都行。
因为than既可以作连词，也可以做介词

例子：
I am taller than him
I do the homework more quickly than he does.

这里的those被视为是主语的时候，需要加do
被视为宾语的时候不需要do.

所以这里不是考点。