A. The birds can fly upward to the air. B. The birds are able to use the upward airstream. C. The birds can create airstream using their wingtips. D. The birds more rely on the front leading birds.
A. The birds can fly upward to the air.
B. The birds are able to use the upward airstream.
C. The birds can create airstream using their wingtips.
D. The birds more rely on the front leading birds.