大全32 if x do，y will do

32.    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
答案没有问题，选E。有问题的是E中是 if x do sth， y do sth ，为什么不是y will do sth，
这里是意思要求，if  calves do....they ...., they 还指代前面从句的主语

答案中解释的是 exercise 和 gain 都用的是一般现在时。但原句中的gain 前的will 不可以看做是省略了吗？貌似有点映象是助动词在什么什么情况下都不能省，是这里的情况吗？