855 Section 07 Q2

2.     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）

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

答案是E，请问than后面的do难道不需要补出来么？为什么？