单选题：根据下列材料请回答题：Why Humans Walk on Two LegsA team of scientists t

根据下列材料请回答题：
Why Humans Walk on Two Legs
A team of scientists that studied chimpanzees (黑猩猩) trained to use treadmills (跑步机) has gathered new evidence suggesting that our earliest apelike ancestors started walking on two legs because it required less energy than getting around on all fours.
Michael sockol , researcher of UC Davis, worked for two years to find an animal trainer willing to coax (劝诱) adult chimps to walk on two legs and to walk on all fours.
The five chimps also wore face masks used to help the researchers measure oxygen consumption. While the chimps worked out, the scientists collected data that allowed them to calculate which method of locomotion (移动) used less energy and why. The team gathered the same information for four adult humans walking on a treadmill.
The researchers found that human walking used about 75 percent less energy and burned 75 percent fewer calories than quadrupedal and bipedal walking in chimpanzees. They also found that for some but not all of the chimps, walking on two legs was no more costly than on all fours.
"We were prepared to find that all of the chimps used more energy walking on two legs-but that finding wouldn't have been as interesting, Sockol said. "what we found was much more telling. For three chimps, bipedalism was more expensive, but for the other two chimps, this wasn't the case. One spent about the same energy walking on two legs as on all fours The other used less energy walking upright。" These two chimps had different gaits (步法) and anatomy (解剖) than 'their quadrupedal peers.
Taken together, the findings provide support for the hypothesis that anatomical (解剖学的) differences affecting gait existed among our earliest apelike ancestors, and that these differences provided the genetic variation which natural selection could act on when changes in the environment gave bipeds an advantage over quadrupeds.
Fossil and molecular evidence suggests the earliest ancestors of the human family lived in forested areas in equatorial Africa in the late Miocene era (中世纪) some 8 to 10 million years ago, when changes in climate may have increased the distance between food patches~ That would have forced our earliest ancestors to travel longer distances on the ground and favored those who could cover more ground using less energy.
"This isn't the complete answer," Sockol said. "But it's a good piece of a puzzle humans have always wondered about: How and why did we become human? And why do we alone walk on two legs?"Michael Sockol and his team were interested in A．where humans came fro
B．how chimpanzees could be trained to use treadmill
C．when our earliest ancestors began to live in forested areas
D．why our apelike ancestors came to walk on two leg