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.
答案：A
答案句：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.
参考译文：这片湖泊的景观奇特而致命——而更致命的是，全球近75%的火烈鸟都出生在这里。… 湖水腐蚀性极强，能灼伤未适应动物的皮肤和眼睛。然而，火烈鸟是唯一真正在这一切死亡中生存的物种。
定位线索：题干“Lake Natron”→答案句取自第1段“The lake's landscape is bizarre and deadly”及第2段“The water is so corrosive”；文中“unadapted animals”对应选项A“most animals”；文中“only species”对应选项A“uninhabitable for most animals”。
解析：题干问关于Lake Natron我们能了解到什么。答案句指出湖水腐蚀性极强，能灼伤未适应动物的皮肤和眼睛，且火烈鸟是唯一能在此生存的物种，说明大多数动物无法在此生存，对应选项A“simply uninhabitable for most animals”。选项B“little known”无中生有，文中未提及；选项C“breeding ground for a variety of birds”偷换概念，文中仅提到火烈鸟在此繁殖；选项D“ideal habitat for lots of predators”与原文“predators can move briskly”矛盾，且文中未说 predators 以此为理想栖息地。
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
答案：C
答案句：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.
参考译文：全球四分之三的火烈鸟从裂谷的其他盐湖飞来，在水位达到特定高度时出现的盐晶岛上筑巢——水位太高，鸟儿无法筑巢；水位太低，捕食者能迅速穿过湖床发起攻击。当水位达到合适高度时，幼鸟被一条腐蚀性沟渠保护，免受捕食者侵害。
定位线索：题干“specific level”→答案句取自第2段“when the water is at a specific level”；文中“kept safe from predators”对应选项C“stay away from predators”。
解析：题干问火烈鸟只在特定水位筑巢的原因。答案句明确指出“When the water hits the right level, the baby birds are kept safe from predators by a corrosive ditch”，即合适水位能让幼鸟远离捕食者，对应选项C“stay away from predators”。选项A“find safe shelter more easily”范围扩大，文中强调防捕食者而非一般庇护；选项B“grow thick feathers on their feet”无中生有，文中未提及；选项D“get accustomed to the salty water”答非所问，与水位无关。
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.
答案：A
答案句：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.”
参考译文：一些鱼类也在湖中取得了有限的成功，因为流入纳特龙湖的温泉在外围形成了盐度较低的泻湖。三种罗非鱼在那里部分时间繁盛。Harper说：“鱼类在溪流中有避难所，当湖水低、泻湖分离时，它们可以扩展到泻湖中。当湖水高时，所有泻湖连在一起，鱼类必须退回溪流避难所，否则就会死亡。”
定位线索：题干“certain species of tilapia”→答案句取自第4段“Three species of tilapia”；文中“less salty lagoons”及“refuge in the streams”对应选项A“take refuge in the less salty waters”。
解析：题干问某些罗非鱼为何有时能在纳特龙湖生存。答案句指出温泉形成盐度较低的泻湖，鱼类在溪流中有避难所，可扩展到泻湖，对应选项A“they can take refuge in the less salty waters”。选项B“flee quick enough from predators”无中生有，文中未提及捕食者；选项C“move freely from lagoon to lagoon”偷换概念，文中说泻湖分离时才能扩展，且湖水高时需退回；选项D“stand the heat of the spring water”无中生有，文中未提及水温问题。
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：D
答案句：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.
参考译文：这个独特的生态系统可能很快面临压力。坦桑尼亚政府再次开始开采湖中的苏打灰，用于制造化学品、玻璃和洗涤剂。尽管计划中的作业地点位于40多英里外，通过管道抽取苏打灰，但环保人士担心它仍可能扰乱自然水循环和繁殖地。
定位线索：题干“planned operation”→答案句取自第5段“planned operation”；文中“upset the natural water cycle and breeding grounds”对应选项D“disruption of Lake Natron's ecosystem”。
解析：题干问坦桑尼亚政府计划作业的后果。答案句指出环保人士担心该作业可能扰乱自然水循环和繁殖地，即破坏生态系统，对应选项D“the disruption of Lake Natron's ecosystem”。选项A“accelerated extinction of flamingos”程度过重，文中只说“under pressure”和“upset”，未提及灭绝；选项B“change of flamingos' migration route”无中生有，文中未提及迁徙路线；选项C“overmining of Lake Natron's soda ash”偷换概念，文中说开采苏打灰是目的，非后果。