求解：一道语法模拟题

A study on couples' retirement transitions found that women who took new jobs after retiring from their primary careeerss 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.

D. which was more so than those retiring.

E, which was more than if they had retired.

答案是C，求nn思路，实在不解more so than 的用法，先谢了。