Dr Katz Is Researching the Causes of Phobias

Dr. Katz is researching the causes of phobias. He puts an ad in a newspaper asking for people who have an intense, distressing fear of snakes to come and participate in his study. The major problem with this is

A) he doesn't know if people are telling the truth about their fears or not.
B) his sample will be too small.
C) he is not getting a representative sample.
D) the people who come may not have a phobia.