Dr Katz Is Researching the Causes of All Phobias

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

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