各位老师好！

A study on couples' retirement transitions found that women who took new jobs after retiring from their primary careers reported high marital satisfaction, more than when retiring completely. 

(A) more than when retiring 

(B) more than if they were to retire 

(C) more so than those who retired 

这是一道GMAT题，答案为C。请问so这里的词性是什么？有省略什么成分吗？这道题的两个比较对象是什么？感谢各位老师。