Answer:
D
Explanation:
If im wrong i am so sorry, have a great day!
These activities are appropriate because they make the leg muscles more flexible.
I think you mean "natural" the answer is Puberty. :D
The upstroke, when the wing is partially flexed also creates lift, but arrests forward movement. As a general rule the slower a bird is flying, the further it'll push it's wings forward in the down-stroke to generate extra lift and the faster a bird is flying the less it'll push it wings forward.