[求助]关于which指代问题的一题

In large doses,analgesics that work in the brain as antagonists to certain chemicals have caused psychological disturbances in patients,whch may limit their potential to relieve severe pain.

(A)which may limit their potential to relieve

(B)which may limit their potential for relieving

(C)which may limit such analgesics' potential to relieve

(D)an effect that may limit their potential to relieve

(E)an effect that may limit potential of such analgesics for relieving

我看的答案是D，解释时说which与先行词被分隔，容易产生歧义。

但我想选C，因为which不会修饰patients，那么应该最近修饰psychological disturbances，好像没什么歧义...

有谁可以告诉我这题的合理解释吗？多谢！

楼主可以参看一下关于which的超长讨论贴http://forum.chasedream.com/dispbbs.asp?boardid=23&replyid=493160&id=53201&page=1&skin=0&Star=10