【答案解析】[听力原文]39-43
Today, I'd like to talk about some of the changes land can undergo, specifically desertification, that's the process through which land becomes part of a desert. Now a desert is defined as a place that receives a certain maximum amount of rainfall. But you may not know that it usually takes more than just a lack of water to turn productive land into a desert. There are several specific human activities that when combined with a lack of rainfall encourage desertification. For example, over cultivation, growing more crops than soil can support. The soil loses its nutrients, so it needs either to be fertilized or to be left unused for at least a season. But if neither of these things happens, if these nutrients in the soil don't get replaced, the damaged soil stops producing. Another cause of desertification is overgrazing. That's when the grasses and trees and shrubs of an area are expected to feed more animals than they reasonably can. Too many animals eating in the same area will kill the vegetation. And because it's the roots of this vegetation that hold much of the soil together, when too much of the vegetation dies, the soil erodes. But maybe the most ironic example of human behavior that can lead to desertification is irrigation. It may seem to run counter to common sense to say that introducing water into an area can cause it to become more like a desert. But there are plenty of bad irrigation practices that do just that. Bringing in too much salty water and then not providing adequate drainage for it will fill the soil with salt, and turn the area into a desert.
39. What is the main topic of the talk?
【正确答案】
D
【答案解析】[听力原文]
How is the soil damaged by over cultivation?
【正确答案】
A
【答案解析】[听力原文]
What is over grazing?
【正确答案】
B
【答案解析】[听力原文]
How does over grazing damage soil?
【正确答案】
C
【答案解析】[听力原文]
According to the speaker, what is the main problem with irrigation?
【正确答案】
A
【答案解析】[听力原文]44-47
I've mentioned how DNA have solved many mysteries in biology. And today I want to talk about how it might relate to hypothesis about the travels of the green turtle. Every winter some green turtles make a 2000 km journey from Brazil to Ascension Island in the middle of Atlantic, where they mate and lay eggs. But the question is why do they travel so far to lay their eggs? One researcher hypothesized that there are two parts to the explanation. One is natal homing, the instinct that drives green turtles to always return to the beach where they were hatched. The second part has to do with continental drift, the theory that the positions of earth continents have changed considerably overtime. Brazil and Ascension Island were once much closer together, and continental drift drove them apart. But the turtles kept on going back to the island where they hatched. However another scientist question this explanation on the ground that it would be very unlikely that conditions would allowe generations of turtles over hundreds of millions of years to keep going back to the same nesting ground every single year. So, what is the connection to DNA? Well, there are groups of green turtles that nest in locations other than the Ascension Island. If green turtles always return to the place where they were hatched, then the turtles that have been going to the Ascension Island to nest would've been genetically isolated long enough to have DNA that was very different from the green turtles that nest else where. But when scientists examine DNA from these turtles, their DNA wasn't that different from the DNA of the turtles that go to Ascension Island. Do you have a shock? Well, we still don't know the answer to the question about why a certain group of turtles go to Ascension Island, but this study was a nice example of the usefulness of DNA analysis to biology.
44. What does the Prof mainly discuss?
【正确答案】
B
【答案解析】[听力原文]
What do green turtles do when they get to Ascension Island?
【正确答案】
B
【答案解析】[听力原文]
According to one hypothesis, why do green turtles go to Ascension Island?
【正确答案】
D
【答案解析】[听力原文]
How is DNA used in research on green turtles?
【正确答案】
B
【答案解析】[听力原文]48-50
As I am sure you are aware, history is full of people who were so admired that over the centuries they become almost mythical figures. George Washington is a good example. Everyone knows this story about his chopping down the cherry tree when he was young and bravely confessing to his mischief later. People greatly admired Washington's integrity. And so, out of that, this story evolved, even though no one knows for sure whether the incident ever occurred. Then there is the American Indian, Poke Hunters, beloved by history for making peace between the English Colonies and the American Indians. The history of her life has also become somewhat mythical. At the historical society exhibit next week, we'll see many artistic works depicting the major events of her life. And while we are there, keeping in mind that much of what you'll see in the paintings reflects how much she was admired, but not necessarily the fact of her life. For instance, one painting shows her saving the life of Jon Smith, an English Colonist, who had been captured by her tribe. Smith, so the story goes, was about to be executed when twelve-years-old Poke Hunters lay her head on top of his. Tells you a lot about her courage. But Jon Smith himself related this story only years after Poke Hunters had become famous, which suggests that he may have embellished the truth a little bit, as many of the works that we'll see in next week may have done. Something else to remember: paintings portrait her physical appearance in many different ways, but always flattening ways. Yet only one picture of her was ever painted while she was living.
48. Why does the speaker mention the story about George Washington and the cherry tree?
【正确答案】
D
【答案解析】[听力原文]
According the speaker, why is Poke Hunters famous?
【正确答案】
D
【答案解析】[听力原文]
What does the speaker imply about most of the paintings of Poke Hunters?