Twoopposing scenarios, the “arboreal” hypothesis and the “cursorial” hypothesis,have traditionally been put forward concerning the origins of bird flight. The“arboreal” hypothesis holds that bird ancestors began to fly by climbing treesand gliding down from branches with the help of incipient feathers: the heightof trees provides a good starting place for launching flight, especiallythrough gliding. As feathers became larger over time, flapping flight evolvedand birds finally became fully air-borne. This hypothesis makes intuitive sense,but certain aspects are troubling. Archaeopteryx (the earliest known bird) andits maniraptoran dinosaur cousins have no obviously arboreal adaptations, suchas feet fully adapted for perching. Perhaps some of them could climb trees, butno convincing analysis has demonstrated how Archaeopteryx would have bothclimbed and flown with its forelimbs, and there were no plants taller than afew meters in the environments where Archaeopteryx fossils have been found. Evenif the animals could climb trees, this ability is not synonymous with glidingability. (Many small animals, and even some goats and kangaroos, are capable ofclimbing trees but are not gliders.) Besides, Archaeopteryx shows no obviousfeatures of gliders, such as a broad membrane connecting forelimbs and hindlimbs.
The“cursorial”(running) hypothesis holds that small dinosaurs ran along the groundand stretched out their arms for balance as they leaped into the air afterinsect prey or, perhaps, to avoid predators. Even rudimentary feathers onforelimbs could have expanded the arm’s surface area to enhance lift slightly.Larger feathers could have increased lift incrementally, until sustained flightwas gradually achieved. Of course, a leap into the air does not provide the accelerationproduced by dropping out of a tree; an animal would have to run quite fast totake off. Still, some small terrestrial animals can achieve high speeds. Thecursorial hypothesis is strengthened by the fact that the immediate theropoddinosaur ancestors of birds were terrestrial, and they had the traits neededfor high lift off speeds: they were small, agile, lightly built, long-legged, andgood runners. And because they were bipedal, their arms were free to evolveflapping flight, which cannot be said for other reptiles of their time.
Q 5: Which of the following is included in the discussion of the cursorial hypothesis but not in the discussion of the arboreal hypothesis?
A.A discussion of some of the features of Archaeopteryx
B.A description of the environment known to have been inhabited by bird ancestors
C.！A possible reason why bird ancestors might have been engaging in activities that eventually evolved into flight
D.A description of the obvious features of animals with gliding ability
E.An estimate of the amount of time it took for bird ancestors to evolve the kind of flapping flight that allowed them to become completely airborne
为什么选择C ，感觉C选项意思太隐晦了~谢谢牛人帮助了！
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