What exactly are white lions?"They are a color variation of regular lions,"says William
Swanson,director of animal research at the Cincinnati Zoo,in Ohio.
     White lions are the result of a rare color gene mutation(变异)．When both a male and
a female lion carry the same white genes,there is a good chance that one or more of their
cubs(幼兽)will be born with white fur.
     Not many lions carry the white mutation.In nature,it's rare.A mutation can make an
animal's survival more difficult.For example,being white makes it harder for the animal to
mix with its surroundings.Mutations also can cause physical problems,so those animals
generally do not survive long enough to reproduce.
     But sometimes white lion cubs are born among wild lions,especially in two areas of
South Africa.
     In 1975 two white cubs were spotted in the wild in South African reserve.They were
seized and sent to a zoo.Today about 500 cubs of those two white lions and a few others
known to have the white color gene live around the world in zoos,circuses,farms and private
preserves.
      White lions may be extremely rare in the wild,but breeders(饲养动物的人)know how
to create more of them.This has created serious concern and debate.
      White lions continue to exist only because they are inbred by people-fathers and
daughters,sisters and brothers.Scientists worry that it also will cause health issues for white
lions.
     Inbreeding to create white lions as moneymaking attractions is only one concern.Another
concern is that some people want to release captive－bred(人工繁殖的)white lions into
the wild in South Africa to restore balance to the ecosystem.However,not everyone agrees
that releasing these white lions into the wild is a good idea.Dan York of Hillsdale College
has been studying white lion genetics and is worried.He says that because so many of today's
white lions are inbred,it could spread health problems.If they breed with wild lions,they'll
pass on the genetic problems to the wild population.
     Instead of releasing captive－bred white lions,York suggests that South Africa concentrate
on protecting all of its wild lions.That would ensure the safety of the wild population of lions,
which may produce healthy white cubs in the future.

1．It'sdifficultforawhiteliontosurvivebecause__________.
a．itsfurcontrastssharplywiththesurroundings
b．themutationmaycausephysicalproblems
c．itislikelytobeattackedbywildlions
d．itishuntedbyhumanbeings

A．a,b　　　　　　　
B．a,d
C．a,c
D．b,d