Which of the Following Is the Most Significant Reason Why

Which of the following is the most significant reason why atypical sexual behaviours have not been studied very widely?

A) They often involve illegal activities that people do not wish to admit to.
B) They are very rare.
C) Researchers have not developed criteria for diagnosing a paraphilia disorder.
D) Researchers are embarrassed to discuss paraphilias with people with paraphilic disorders.