Chimpanzees, long considered reluctant to share, apparently can display selfless tendencies, revealing one more key way our closest living relatives are like humans, scientists find. These findings could shed light on the evolution of _ in humans. Previously, a belief was widely held that human altruism evolved only after humans split from their ape cousins about 6 million years ago. In recent years, however, research has revealed just how much chimpanzees have in common with us. They can hunt with spears, play with dolls and mourn their partners' death. "Most of the earlier studies had presented the apes with a complex implement that helped them deliver food to themselves or others, often so complicated that the experiments tested tool skills rather than social tendencies," De Waal told Live Science. "Ours is the first study that uses no such implement at all." In addition to using complex food-delivery systems, past experiments often placed the chimpanzees so far apart that they might not have realized how their actions benefited others. In these new, simplified experiments, two apes were housed next to each other with a screen through which they could see each other. Then, one chimpanzee had to choose between two differently colored tokens   from a bin, one of which represented a pro-social   option, the other a selfish option. The pro-social option would cause both chimpanzees to receive a piece of banana wrapped in paper. The selfish option only rewarded the ape who made the choice. In a study with seven adult female chimps placed into various pairs, the scientists found all the apes showed a definite preference for the pro-social option. "For me, the most important finding is that like us, chimpanzees take into account the needs and wishes of others," researcher Victoria Horner, said. The chimpanzees behaved especially altruistically toward partners who either patiently waited or gently reminded them that they were there by drawing attention to themselves. They were less likely to reward partners who pressured through making a fuss, begging persistently or spitting water at them. This is opposite to a long-standing view that the chimpanzees only share food under pressure. Recent researches are performed with less complicated implements mainly to   _  .
show that chimps are only good at using implements see whether chimps are willing to share food with others find out if chimps are likely to reward partners test chimps' social tendencies instead of tool skills
Question: Chimpanzees, long considered reluctant to share, apparently can display selfless tendencies, revealing one more key way our closest living relatives are like humans, scientists find. These findings could shed light on the evolution of _ in humans. Previously, a belief was widely held that human altruism evolved only after humans split from their ape cousins about 6 million years ago. In recent years, however, research has revealed just how much chimpanzees have in common with us. They can hunt with spears, play with dolls and mourn their partners' death. "Most of the earlier studies had presented the apes with a complex implement that helped them deliver food to themselves or others, often so complicated that the experiments tested tool skills rather than social tendencies," De Waal told Live Science. "Ours is the first study that uses no such implement at all." In addition to using complex food-delivery systems, past experiments often placed the chimpanzees so far apart that they might not have realized how their actions benefited others. In these new, simplified experiments, two apes were housed next to each other with a screen through which they could see each other. Then, one chimpanzee had to choose between two differently colored tokens   from a bin, one of which represented a pro-social   option, the other a selfish option. The pro-social option would cause both chimpanzees to receive a piece of banana wrapped in paper. The selfish option only rewarded the ape who made the choice. In a study with seven adult female chimps placed into various pairs, the scientists found all the apes showed a definite preference for the pro-social option. "For me, the most important finding is that like us, chimpanzees take into account the needs and wishes of others," researcher Victoria Horner, said. The chimpanzees behaved especially altruistically toward partners who either patiently waited or gently reminded them that they were there by drawing attention to themselves. They were less likely to reward partners who pressured through making a fuss, begging persistently or spitting water at them. This is opposite to a long-standing view that the chimpanzees only share food under pressure. Recent researches are performed with less complicated implements mainly to   _  .
A. show that chimps are only good at using implements
B. see whether chimps are willing to share food with others
C. find out if chimps are likely to reward partners
D. test chimps' social tendencies instead of tool skills
Answer: