If Flight Is Such an Advantage,how Do We Account for the Evolution

If flight is such an advantage,how do we account for the evolution of flightless birds?

A) They were primitive ancestors of the winged birds
B) Flight is costly in terms of energy so birds abandon flight if they live in areas with food and without substantial predators,such as islands
C) This is always an inferior situation if they can't fly,so wingless birds occur only where winged birds are missing
D) Birds give up flight if they can avoid having to lay eggs
E) None of the choices is correct

Correct Answer:

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