请教:OG-紫色—39

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 quiker than do those raised in confinement

答案：associate with other calves, they require less medication and gain weight more quickly than

答案说do语法错误且影响比较结构

可是很多类似的题目都要补do

到底什么时候该补什么时候不该补？？