一道真題 cost n用法 用lower or less?

[HCA]

一道真題
Employment costs rose 2.8 percent in the 12 months that ended in September, slightly [u]less than they did [/u]in the year that ended in the previous quarter.
A less than they did
B less than it ded
C less than they were
D lower than
E lower than they were

答案是A
可以這樣解釋嗎？
比較時用 low employment cost 即可去對應 high employment cost
但在比較級時則需用less