Scientists have been puzzled by the phenomenon that migrating birds fly not just in groups all the time, but in "V" formations and they have tried to figure out what benefit birds get from this particular formation. Now, a research group from University of London may have found the answer -- migrating birds fly in a "V" to save energy, according to a study published in the journal Nature. When a bird flaps  its wings, it stirs the air around it and causes it to move indifferent directions. Scientist found in the study that the air creates an upward-moving wave at the tips of the wings, which means that if a bird flies around the wingtips of another bird, it can get a boost  from the rising air and therefore use less energy to stay in the air. Apart form the "free ride" strategy, scientists were also surprised to observe that migrating birds timed their wing beats and adjusted their positions in a very precise way to maximize the lifting effect while avoiding areas where the air move downward. "They're able to sense what's going on from the bird in front, where this 'good air' is coming from and now how to position themselves perfectly in it," lead researcher Steven Portugal told BBC. In fact, the "V" formation has long thought to help birds fly more efficiently . A previous study showed that birds' heart rates went down when they were flying together in a "V" formation. Jet fighters were also found to be able reduce their energy use by up to 18% by staying near the wingtips of other jet fighters. Both of these findings led scientists to suspect that the "V" formation had an efficiency purpose, but until now they still lack proof. "For scientists, the new study provides an insight into an interesting natural phenomenon. But it could mean even more for aircraft companies -- helping them understand how they can _ that with their plane formations to save fuel, said Portugal." According to the recent study, when birds migrate  _  .
they don't change their lead birds during the whole journey they fly in "V" formations to protect themselves against enemies they form the shape of a "V" to make best use of the lifting effect the air they stir around makes it hard for the birds behind to fly fast
Question: Scientists have been puzzled by the phenomenon that migrating birds fly not just in groups all the time, but in "V" formations and they have tried to figure out what benefit birds get from this particular formation. Now, a research group from University of London may have found the answer -- migrating birds fly in a "V" to save energy, according to a study published in the journal Nature. When a bird flaps  its wings, it stirs the air around it and causes it to move indifferent directions. Scientist found in the study that the air creates an upward-moving wave at the tips of the wings, which means that if a bird flies around the wingtips of another bird, it can get a boost  from the rising air and therefore use less energy to stay in the air. Apart form the "free ride" strategy, scientists were also surprised to observe that migrating birds timed their wing beats and adjusted their positions in a very precise way to maximize the lifting effect while avoiding areas where the air move downward. "They're able to sense what's going on from the bird in front, where this 'good air' is coming from and now how to position themselves perfectly in it," lead researcher Steven Portugal told BBC. In fact, the "V" formation has long thought to help birds fly more efficiently . A previous study showed that birds' heart rates went down when they were flying together in a "V" formation. Jet fighters were also found to be able reduce their energy use by up to 18% by staying near the wingtips of other jet fighters. Both of these findings led scientists to suspect that the "V" formation had an efficiency purpose, but until now they still lack proof. "For scientists, the new study provides an insight into an interesting natural phenomenon. But it could mean even more for aircraft companies -- helping them understand how they can _ that with their plane formations to save fuel, said Portugal." According to the recent study, when birds migrate  _  .
A. they don't change their lead birds during the whole journey
B. they fly in "V" formations to protect themselves against enemies
C. they form the shape of a "V" to make best use of the lifting effect
D. the air they stir around makes it hard for the birds behind to fly fast
Answer: