21. In the opening paragraph, the author introduces his topic by

　　[A] posing a contrast.

　　[B] justifying an assumption.

　　[C] making a comparison.

　　[D] explaining a phenomenon.

　　22. The statement “it is all too monkey” (Last line, Paragraph 1) implies that

　　[A] monkeys are also outraged by slack rivals.

　　[B] resenting unfairness is also monkeys’ nature.

　　[C] monkeys, like humans, tend to be jealous of each other.

　　[D] no animals other than monkeys can develop such emotions.

　　23. Female capuchin monkeys were chosen for the research most probably because they are

　　[A] more inclined to weigh what they get.

　　[B] attentive to researchers’ instructions.

　　[C] nice in both appearance and temperament.

　　[D] more generous than their male companions.

　　24. Dr. Brosnan and Dr. de Waal have eventually found in their study that the monkeys

　　[A] prefer grapes to cucumbers.

　　[B] can be taught to exchange things.

　　[C] will not be co-operative if feeling cheated.

　　[D] are unhappy when separated from others.