A、The anger of a relative, friend or enemy. B、The stone hidden inside the patient’s throat, arm, leg, stomach, etc. C、The attack

Scientists are fascinated by what they learn from the methods of traditional healers, people who cure patients through means other than prescribing medicines. The question that scientists ask is how traditional healers can actually be successful in their treatments and why they are so popular.
    The Ndembu people of Central Africa, for example, believe that illness is often the result of the anger of a relative, friend, or enemy. This emotion, they say, causes a tooth to enter the body of the person who is the target of the anger and to create disease.
    When the healer decides which body part contains the evil tooth, he calls together the victim’s relatives and friends to watch a ceremony, at the end of which he "removes" it from the patient’s throat, arm, leg, stomach, etc. Although the patient and the villagers know what has happened — that the tooth has been hidden inside the healer’s mouth the whole time, the patient is often cured.
    The Ndembu ceremony is typical of treatments around the world, in which stones, insects, or small sharp objects are "taken out" of sick people. Despite scientific evidence that disproves the effectiveness of such methods, the fact remains that the treatments are often successful, and many villagers prefer to be cured in this way than by a modern physician.
Questions 20 to 22 are based on the passage you have just heard.
20. Which aspect of traditional healers do scientists find difficult to understand?
21. In view of Ndembu people, what is often the cause of people’s illness?
22. What do many villagers think about the healers’ methods?