一道语法题求指教

sfyy

题目：
In his research paper, Dr. Frosh, medical director of the Payne Whitney Clinic, distinguishes [mood swings, which may be violent without their being grounded in mental disease, from genuine manic-depressive psychosis.]
画线部分为[]中的部分，

A. mood swings, which may be violent without their being grounded in mental disease, from genuine manic-depressive psychosis （即原句）
C. between mood swings, which may be violent without being grounded in mental disease, and genuine manic-depressive psychosis.

答案：C

请问A选项中without their being 是错的吗？我记得动名词有个复合结构，介词+物主代词+动名词，这里为什么错呢？
C选项中将distinguish...from...换成了distinguish between...and... 这两个短语可以互相替代吗？我觉得distinguish from好像更侧重前者，而distinguish between就两者侧重程度相同了。

谢谢！

godiva11

我看到有人对distinguish between...and ... 和distinguish ...from...的解释是：
从语法上讲distinguish A from B 和 distinguish between A and B 是都对的
但是一般distinguish A from B 这个A 和 B要是完全对立的两个东西；而这里mood swings和genuine manic-depressive psychosis都是两种情绪，并没有完全对立。
而且，一种说法就是ETS更加倾向于distinguish between A and B