Passage One
Questions 46 to 50 are based on the following passage.
At the base of a mountain in Tanzania's Gregory Rift, Lake Natron burns bright red, surrounded by the remains of animals that were unfortunate enough to fall into the salty water. Bats, swallows and more are chemically preserved in the pose in which they perished, sealed in the deposits of sodium carbonate in the water. The lake's landscape is bizarre and deadly—and made even more so by the fact that it's the place where nearly 75 percent of the world's flamingos (火烈鸟)are born.
The water is so corrosive that it can burn the skin and eyes of unadapted animals. Flamingos, however, are the only species that actually makes life in the midst of all that death. Once every three or four years, when conditions are right, the lake is covered with the pink birds as they stop flight to breed. Three-quarters of the world's flamingos fly over from other salt lakes in the Rift Valley and nest on salt-crystal islands that appear when the water is at a specific level—too high and the birds can't build their nests, too low and predators can move briskly across the lake bed and attack. When the water hits the right level, the baby birds are kept safe from predators by a corrosive ditch.
"Flamingos have evolved very leathery skin on their legs so they can tolerate the salt water," says David Harper, a professor at the university of Leicester. "Humans cannot, and would die if their legs were exposed for any length of time." So far this year, water levels have been too high for the flamingos to nest.
Some fish, too, have had limited success vacationing at the lake as less salty lagoons (泻湖) form on the outer edges from hot springs flowing into Lake Natron. Three species of tilapia (罗非鱼) thrive there part-time. "Fish have a refuge in the streams and can expand into the lagoons when the lake is low and the lagoons are separate," Harper said. "All the lagoons join when the lake is high and fish must retreat to their stream refuges or die." Otherwise, no fish are able to survive in the naturally toxic lake.
This unique ecosystem may soon be under pressure. The Tanzanian government has once again started mining the lake for soda ash, used for making chemicals, glass and detergents. Although the planned operation will be located more than 40 miles away, drawing the soda ash in through pipelines, conservationists worry it could still upset the natural water cycle and breeding grounds. For now, though, life prevails—even in a lake that kills almost everything it touches.
46. What can we learn about Lake Natron?
A) It is simply uninhabitable for most animals.
B) It remains little known to the outside world.
C) It is a breeding ground for a variety of birds.
D) It makes an ideal habitat for lots of predators.
47. Flamingos nest only when the lake water is at a specific level so that their babies can .
A) find safe shelter more easily
B) grow thick feathers on their feet
C) stay away from predators
D) get accustomed to the salty water
48. Flamingos in the Rift Valley are unique in that .
A) they can move swiftly across lagoons
B) they can survive well in salty water
C) they breed naturally in corrosive ditches
D) they know where and when to nest
49. Why can certain species of tilapia sometimes survive around Lake Natron?
A) They can take refuge in the less salty waters.
B) They can flee quick enough from predators.
C) They can move freely from lagoon to lagoon.
D) They can stand the heat of the spring water.
50. What may be the consequence of Tanzanian government's planned operation?
A) The accelerated extinction of flamingos.
B) The change of flamingos' migration route.
C) The overmining of Lake Natron's soda ash.
D) The disruption of Lake Natron's ecosystem.
答案解析：
46. 由题干中的关键词“Lake Natron”定位到第一段。第一段提到“The lake's landscape is bizarre and deadly—and made even more so by the fact that it's the place where nearly 75 percent of the world's flamingos (火烈鸟)are born.”以及第二段“The water is so corrosive that it can burn the skin and eyes of unadapted animals.”，说明纳特龙湖对大多数不适应的动物来说是致命的，不可居住的。选项A“它对大多数动物来说根本不适合居住”符合文意，所以选A。
47. 由题干中的关键词“Flamingos nest”“lake water is at a specific level”定位到第二段。第二段提到“...nest on salt - crystal islands that appear when the water is at a specific level—too high and the birds can't build their nests, too low and predators can move briskly across the lake bed and attack. When the water hits the right level, the baby birds are kept safe from predators by a corrosive ditch.”，说明火烈鸟只在湖水达到特定水位时筑巢，这样雏鸟能远离捕食者。选项C“远离捕食者”符合文意，所以选C。
48. 由题干中的关键词“Flamingos in the Rift Valley”“unique”定位到第二段和第三段。第二段提到火烈鸟在致命的湖水中生活，第三段提到“Flamingos have evolved very leathery skin on their legs so they can tolerate the salt water”，说明东非大裂谷的火烈鸟独特之处在于能在咸水中生存。选项B“它们能在咸水中很好地生存”符合文意，所以选B。
49. 由题干中的关键词“certain species of tilapia”“survive around Lake Natron”定位到第四段。第四段提到“Some fish, too, have had limited success vacationing at the lake as less salty lagoons (泻湖) form on the outer edges from hot springs flowing into Lake Natron. Three species of tilapia (罗非鱼) thrive there part - time.”，说明某些罗非鱼能在纳特龙湖周围生存是因为它们可以在盐度较低的泻湖中避难。选项A“它们可以在盐度较低的水域避难”符合文意，所以选A。
50. 由题干中的关键词“Tanzanian government's planned operation”定位到最后一段。最后一段提到“Although the planned operation will be located more than 40 miles away, drawing the soda ash in through pipelines, conservationists worry it could still upset the natural water cycle and breeding grounds.”，说明坦桑尼亚政府计划的开采苏打灰行动可能会破坏纳特龙湖的自然水循环和繁殖地，即破坏其生态系统。选项D“纳特龙湖生态系统的破坏”符合文意，所以选D。
