A Few Birds,such as Ostriches,have Wings but Cannot Fly

A few birds,such as ostriches,have wings but cannot fly.Which of the following best explains this?

A) Ostriches are not true birds.
B) Ostriches evolved from an amphibian ancestor that had winglike arms and glided among trees.
C) When reptiles evolved into birds,some birds developed wings for flight,but others developed wings incapable of flight.
D) Ostriches evolved from a bird ancestor that flew.
E) Ostriches evolved first,then all other flying birds evolved from ostriches.