Which of the following statements is correct regarding the evolution of flight?
a. the evolution of feathers on wings was initially to support fight in non-avian dinosaurs
b. the bottom-up theory postulates that the evolution of flight started with the use of wings to glide from branch to branch.
c. the top-down theory postulates that the evolution of flight started with flapping to aid in generating lift off the ground.
d. the wing-assisted incline running theory postulates that wings were initially used to maintain balance while climbing up slopes.

d.The wing-assisted incline running theory postulates that wings were initially used to maintain balance while climbing up slopes.

Explanation:

Two theories were given to explain the origin of avian flight. The top-down theory suggested that the birds' ancestors were tree dwelling species. They took help of wings to glide from tree to tree in order to avoid predator or catch prey. The bottom up theory suggested that the birds' ancestors were ground dwelling species. They took help of wings to maintain a balance and provide a lift for efficient running.

Wing-assisted incline running was a mixture of both the above theories and gave a balanced view point. According to it, birds' ancestors used wings to maintain a balance while running up inclined slopes. The wings helped them to save energy while scaling slopes and eventually a flight mechanism was also developed.

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