Why Is It Difficult to Describe Effective Treatments for Conversion

Why is it difficult to describe effective treatments for conversion disorder?

A) Extensive research has found drugs work for some patients while cognitive-behavioural therapy works for others.
B) Extensive research has found that there are no effective treatments for the disorder.
C) Research in the 1970s suggested drug treatments worked; however, recent research suggests that drug treatments do not work.
D) There have been very few systematic, controlled studies evaluating treatment.