15. 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.

(A) mood swings, which may be violent without their being grounded in mental disease, from genuine manic-depressive psychosis

(B) mood swings, perhaps violent without being grounded in mental disease, and genuine manic-depressive psychosis ,

(C) between mood swings, which may be violent without being grounded in mental disease, and genuine manic-depressive psychosis

(D) between mood swings, perhaps violent without being grounded in mental disease, from genuine manic-depressive psychosis

(E) genuine manic-depressive psychosis and mood swings, which may be violent without being grounded in mental disease

The best choice is C because it uses the idiomatically correct expression distinguishes between x and y and because it provides a structure in which the relative clause beginning which may be violent clearly modifies mood swings. The other choices use distinguishes in unidiomatic constructions. Additionally, their in A is intrusive and unnecessary, and the modifier of mood swings in B and D (perhaps violent) is awkward and less clear than the more developed clause which may be violent.

请教NN们，我咋就不觉得viloent unclear呢，modifier什么时候用releative clause，什么时候可以用 adj.啊

choice B. simplify the sentence, we get:

Dr. F distinguishes mood swings, perhaps violent, and ...

You see, "perhaps violent" can modify Dr. F and mood swings.

That's why ETS uses 'less clear'.