新版prep part 1 220, after falling为什么是状语从句省略？各位NN，帮帮忙~~

湿泪的手

220. (GWD-24-8, OG12-86)The results of the company's cost-cuttingmeasures are evident in its profits, which increased five percent during thefirst three months of this year after it fell over the last two years.  
A.      whichincreased five percent during the first three months of this year after it fell
B.      whichhad increased five percent during the first three months of this year after ithad fallen
C.      whichhave increased five percent during the first three months of this year afterfalling
D.       with a five percent increase during the firstthree months of this year after falling  (C)
E.  with a five percent increase during the firstthree months of this year after having fallen

答案选C，但解释上有点问题我不明白：下面是语法笔记上对于B和C的解释：
B. after it had fallen时态上没有问题，可以与over the last two years搭配，但是不如after falling简洁
C. Correct；afterfalling简洁，状语从句的省略

上面两个解释，我理解的就是afterfalling就是afterit had fallen的省略形式？可是状语从句省略的一个条件不是状从的谓语动词是be么，本题这样也是可以省略的吗？