What Is One Problem with Past Research on Paraphilias

What is one problem with past research on paraphilias?

A) Most of the research has been done with clinical and incarcerated samples, which are not representative of the general population.
B) Most of the research occurred before 1950; therefore it is not generalizable to today's population.
C) Most of the research has focused only on non-consenting paraphilias thus not giving a broad overview of the range of paraphilias that exist.
D) Most of the research has been done with people who were voluntarily seeking treatment for their paraphilia, thus creating volunteer bias in the findings.