完形填空（共20小题；每小题1分，满分20分）
请认真阅读下面短文，从短文后各题所给的A、B、C、D四个选项中，选出最佳选项，并在答题卡上将该项涂黑。
Most people think of birds as feathered animals that fly. Scientists,   36  , do not define (给…下定义) birds as animals that fly, because some birds do not. Of the 10,000 or so species of birds, 46 cannot   37  . Flight plays a key role in   38   for most birds. It allows them to find food over a   39   area and to get away from enemies. Ancestors of flightless birds may have   40    their ability to fly because they had no regular predators (捕食者) or did not need to fly to find food. Rather than fly, some of these birds developed other   41   to catch food and avoid enemies. Two examples of   42   birds are penguins and ostriches.
  43   birds that fly, penguins do not have wide wings or large feathers. In order to catch the fish they feed on, penguins use their powerful wings to swim   44  . And when they swim,  they look as if they are flying through the water. The   45   of penguins’ bodies makes it possible for them to dive deep underwater,  46  their thick feathers protect them from the cold.
Ostriches are known for their long legs, long necks, and large size. To   47   themselves, they stay in groups and use their excellent sight and hearing to   48   enemies. As soon as    49   approaches, they can run at a speed of more than 65 kilometers per hour to   50   . In addition to using their strong legs to run, they can kick powerfully. 
Not all flightless birds have been   51  in protecting themselves. Flightless birds on some islands had no enemies until people   52  . These birds were hunted and easily caught by people and the animals   53   by people. Human land development has   54   the habitats of some birds. A number of flightless birds   55   because they were unable to adapt to new conditions and new enemies.

A．thus B．however C．therefore D．otherwise

A．stand B．fly C．hear D．sing

A．growth B．communication C．health D．survival

A．wide B．familiar C．crowded D．special

A．developed B．improved C．lost D．proved

A．parts B．habits C．ways D．tools

A．wild B．interesting C．rare D．flightless

A．Instead of B．Because of C．Besides D．Unlike

A．quickly B．carefully C．differently D．gradually

A．shape B．color C．bone D．skin

A．if B．and C．but D．so

A．help B．feed C．protect D．hide

A．kill B．notice C．confuse D．frighten

A．dawn B．darkness C．danger D．food

A．look B．escape C．move D．fight

A．successful B．natural C．unusual D．positive

A．realized B．acted C．stopped D．arrived

A．brought B．found C．bought D．hunted

A．provided B．formed C．destroyed D．controlled

A．flew away B．watched out C．gave away D．died out