NARRATOR:Listen to part of a lecture in an earth science class.
MALE PROFESSOR:We're really just now beginning to understand how quickly drastic climate change can take place.We can see past occurrences of climate change that took place over just a few hundred years.
Take uh... the Sahara Desert in Northern Africa. The Sahara was really different 6,000 years ago.I mean, you wouldn't call it a tropical paradise or anything, uh... or maybe you would if you think about how today in some parts of the Sahara it... it only rains about once a century.Um... but basically, you had greenery and you had water.
And what I find particularly interesting and amazing really, what really indicates how un-desert-like the Sahara was thousands of years ago was something painted on the rock, pre-historic art, hippopotamuses.As you know hippos need a lot of water and hence? Hence what?
FEMALE STUDENT:They need to live near a large source of water year round.
MALE PROFESSOR:That's right.
MALE STUDENT:But how is that proved that the Sahara used to be a lot wetter?I mean the people who painted those hippos, well, couldn't they have seen them on their travels?
MALE PROFESSOR:Okay, in principle they could, Karl. But the rock paintings aren't the only evidence.Beneath the Sahara are huge aquifers, basically a sea of fresh water that's perhaps a million years old, filtered through rock layers.And... er... and then there is fossilized pollen, from low shrubs and grasses that once grew in the Sahara.In fact these plants still grow, er... but hundreds of miles away, in more vegetated areas.Anyway, it's this fossilized pollen along with the aquifers and the rock paintings—these three things are all evidence that the Sahara was once much greener than it is today, that there were hippos and probably elephants and giraffes and so on.
MALE STUDENT:So what happened?
MALE PROFESSOR:How did it happen?Well now we're so used to hearing about how human activities are affecting the climate, right; but that takes the focus away from the natural variations in the earth climate, like the Ice Age, right?The planet was practically covered in ice just a few thousand years ago.Now as far as the Sahara goes, there is some recent literature that points to the migration of the monsoon in that area.
FEMALE STUDENT & MALE STUDENT:[audible] “huh?”
MALE PROFESSOR:What do I mean?Okay, a monsoon is a seasonal wind that can bring in a large amount of rainfall.Now if the monsoon migrates, well, that means that the rains move to another area, right?
So what caused the monsoon to migrate?Well, the answer is: the dynamics of earth's motions, the same thing that caused the Ice Age by the way.[slowly, wanting every word to sink in] The earth's not always the same distance from the sun, and it's not always tilting toward the sun at the same angle.There are slight variations in these two parameters.They're gradual variations but their effects can be pretty abrupt, and can cause the climate to change in just a few hundred years.
FEMALE STUDENT:That's abrupt?
MALE PROFESSOR:Well, yeah, considering that other climate shifts take thousands of years, this one is pretty abrupt.So these changes in the planet's motions, they caused the climate to change; but it was also compounded.
[key point] What the Sahara experienced was um... sort of runaway drying effect.As I said the monsoon migrated south—there was less rain in the Sahara.The land started to get drier, which in turn caused huge decrease in the amount of vegetation, because vegetation doesn't grow as well in dry soil, right?And then, less vegetation means the soil can't hold water as well—the soil loses its ability to retain water when it does rain.So then you have less moisture to help clouds form, nothing to evaporate for cloud formation.And then the cycle continues—less rain, drier soil, less vegetation, fewer clouds, less rain etc. etc..
MALE STUDENT:But, what about the people who made the rock paintings?
MALE PROFESSOR:Good question. Well, no one really knows.But there might be some connections to ancient Egypt.At about the same time that the Sahara was becoming a desert... hmm... 5,000 years ago, Egypt really began to flourish out in the Nile River valley.And that's not that far away.So it's only logical to hypothesize that a lot of these people migrated to the Nile valley when they realized that this was more than a temporary drought.And some people take this a step further--and that's okay, that's science--and they hypothesize that this migration actually provided an important impetus in the development of ancient Egypt.[intrigued, not skeptical] Well, we'll stay tuned on that.
We’re really just now beginning to understand how quickly drastic climate change can take place. We can see past occurrences of climate change that took place over just a few hundred years. Take hm the Sahara desert in Northern Africa.
选项分析：教授说要讲how quickly drastic climate change can take place，然后说Take hm the Sahara desert in Northern Africa，其实暗含着for example，只不过没有说出口，对应着答案A。
B选项，comparison未提及。C选项，present-day是细节。D选项，文章讲了recent literature about migration of the monsoon，并没有说geological findings。