题目内容：
第50空______
A．Standing on two legs allows them to carry much more at one time because it frees up their hands B．But an international team of researchers, including Dr. Richmond from GW's Columbian College of Arts and Sciences，have discovered that human walking upright, may have originated millions of years ago as an adaptation to carrying scarce, high- quality resources.
C．Here, 35 percent of the chimpanzees activity involved some sort of bipedal movement, and once again, this behavior appeared to be linked to a clear attempt to carry as much as possible at one time
D．暂无
E．Researchers allowed the wild chimpanzees access to different combinations of two different types of nut — the oil palm nut，which is naturally widely available, and the coula nut, which is not
F．The chimpanzees regarded the coula nuts as a more highly-prized resource and competed for them more intensely
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