Two opposing 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 trees and gliding down from
branches with the help of incipient feathers: the height
of trees provides a good starting place for launching
flight, especially through gliding. As feathers became
larger over time, flapping flight evolved and birds
finally became fully air-borne. This hypothesis makes
intuitive sense, but certain aspects are troubling.
Archaeopteryx (the earliest known bird) and its
maniraptoran dinosaur cousins have no obviously
arboreal adaptations, such as feet fully adapted for
perching. Perhaps some of them could climb trees, but
no convincing analysis has demonstrated how
Archaeopteryx would have both climbed and flown
with its forelimbs, and there were no plants taller than
a few meters in the environments where
Archaeopteryx fossils have been found. Even if the
animals could climb trees, this ability is not
synonymous with gliding ability. (Many small animals,
and even some goats and kangaroos, are capable of
climbing trees but are not gliders.) Besides,
Archaeopteryx shows no obvious features of gliders,
such as a broad membrane connecting forelimbs and
hind limbs.
The "cursorial" (running) hypothesis holds that small
dinosaurs ran along the ground and stretched out their
arms for balance as they leaped into the air after
insect prey or, perhaps, to avoid predators. Even
rudimentary feathers on forelimbs could have
expanded the arm's surface area to enhance lift
slightly. Larger feathers could have increased lift
incrementally, until sustained flight was gradually
achieved. Of course, a leap into the air does not
provide the acceleration produced by dropping out of a
tree; an animal would have to run quite fast to take
off. Still, some small terrestrial animals can achieve
high speeds. The cursorial hypothesis is strengthened
by the fact that the immediate theropod dinosaur
ancestors of birds were terrestrial, and they had the
traits needed for high lift off speeds: they were small,
agile, lightly built, long-legged, and good runners. And
because they were bipedal, their arms were free to
evolve flapping flight, which cannot be said for other
reptiles of their time.

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