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

[spoiler]Manhattan SC quotes the following as the correct usage:
Present: If X happens, Y will happen
Past: If X happened, Y would/could happen
Past Perfect: If X had happened, Y would/could have happened
How come then the correct answer is E? even i chose E compared to others but E should have been will require
pls correct me if I am wrong[/spoiler]