题目内容：
回答题：Researchers Discover Why Humans Began Walking Upright Most of us walk and carry items in our hands every day. These are seemingly simple activities that the majority of us don’t question.__46___The team of researchers from the U. S., England, Japan and Portugal investigated the behavior of modern-day chimpanzees as they competed for food resources，in an effort to understand what ecological settings would lead a large ape — one that resembles the 6 million-year old ancestor we shared in common with living chimpanzees — to walk on two legs.
“These chimpanzees provide a model of the ecological conditions under which our earliest ancestors might have begun walking on two legs, "，said Dr. Richmond.
The research findings suggest that chimpanzees switch to moving on two limbs instead of four in situations where they need to monopolize a resource. 47. Over time，intense bursts of bipedal activity may have led to anatomical changes that in turn became the subject of natural selection where competition for food or other resources was strong.
Two studies were conducted by the team in Guinea. The first study was conducted by the team in Kyoto University’s “ outdoor laboratory ” in a natural clearing in Bossou Forest.___48__. The chimpanzees’ behavior was monitored in three situations:(a) when only oil palm nuts were available,(b)when a small number of coula nuts were available，and(c) when coula nuts were the majority available resource.
When the rare coula nuts were available only in small numbers, the chimpanzees transported more at one time. Similarly, when coula nuts were the majority resource, the chimpanzees ignored the oil palm nuts altogether. __49____.
In such high-competition settings，the frequency of cases in which the chimpanzees started moving on two legs increased by a factor of four. Not only was it obvious that bipedal movement allowed them to carry more of this precious resource, but also that they were actively trying to move as much as they could in one go by using everything available 一 even their mouths.
The second study, by Kimberley Hockings of Oxford Brookes University, was a 14-month study of Bossou chimpanzees crop-raiding, a situation in which they have to compete for rare and unpredictable Resources.___50___.
第46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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