求解一道Prep的比较题1-24

Employment costs rose 2.8 percent in the 12 months that ended in September, slightly less than they did in the year that ended in the previous quarter.

(A) less than they did

(B) less than it did

(C) less than they were

(D) lower than

(E) lower than they were  

为什么划线的成分不可以指代the percentage increase呢？这样的话，就是Employment costs rose 2.8% in the 12 months that ended in September, slightly lower than (the percentage increase) in the year that ended in the previous quarter. 那么D也可以啊...比较题我已经做糊涂了，还望大家指教！

Employment costs rose 2.8 percent in the 12 months that ended in September, slightly less than they did in the year that ended in the previous quarter.

(A) less than they did

(B) less than it did

(C) less than they were

(D) lower than

(E) lower than they were  

为什么划线的成分不可以指代the percentage increase呢？这样的话，就是Employment costs rose 2.8% in the 12 months that ended in September, slightly lower than (the percentage increase) in the year that ended in the previous quarter. 那么D也可以啊...比较题我已经做糊涂了，还望大家指教！