从短文后各题所给的四个选项（A、B、C和D）中，选出可以填入空白处的最佳选项。
Danger-junkie orangutans(猩猩) in Borneo climb dead trees and shake then until they begin to fall. They scream with excitement as they cling to the falling tree. Just before the tree hits the ground the orangutans leap to another tree or vine, narrowly escaping death. While no one cab ask orangutans if they enjoy it as a person playing an extreme sport, one animal behaviorist sees this monkey fun as a bit of harmless thrill-seeking.
A growing number of scientists agree that animals are conscious and capable of experiencing basic emotions(情感), such as happiness, sadness, boredom or depression. A few scientists even see the possibility for higher animal emotions like love, jealousy and spite.
Five years ago, behaviorist and animal-rights activist Dr. Jonathan Balcombe stood on a Virginia hotel balcony watching two crows intimately groom (清洁) each other in the comfort of an abandoned billboard. He felt that the birds liked what they were doing, even if engaged in a natural, beneficial act, such as picking parasites (寄生虫) off the other's feathers. That moment changed the way he would view animals forever and led to a book, “Pleasurable Kingdom: Animals and the Nature of Feeling Good”, which is filled with hundreds of examples of animals living it up thanks to developed senses of touch, taste, sight, sound and smell.
Balcombe recounts a favorite example of Kenyan hippos (河马) having high-end spa treatment in a fresh water spring. They splay their toes, open their mouths wide and wait for a school of cleaner fish to remove parasites and slough off dead skin, he recalls. Balcombe knows that the hippos and the fish both benefit from this arrangement. “My interpretation is that it is also enjoyable for them ,” he says.
