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[TOEFL iBT Listening Tests 2015] NEW TEST 20 - With Answers & Transcripts

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Xuất bản 15/08/2015
Practice these TOEFL iBT listening tests to help you score high in the TOEFL Listening Section. Check the correct answers and audio transcripts below. This video is in the series of NEW TOEFL iBT Tests 2015. Okay, I know from having taught this class before that the topic I’m about to cover has the potential to get people’s blood boiling, so I’d like to start off by making something clear: I’m going to discuss the physical effects of tobacco on the body. This is not the place for a debate over social issues. If you all want to get into it over lunch after class is over, you’re welcome to. Just don’t expect me to join in. Anyway, what we want to talk about is what happens to your body when you smoke or chew tobacco. The primary active ingredient in tobacco is one you’re probably all familiar with: nicotine. Nicotine is a class of chemical called an alkaloid, meaning that it’s a close relative of the caffeine in coffee and the natural drugs found in some plants. It generally has a stimulant effect—it gets your body systems going, makes them more active. It actually makes up a very small percent of the tobacco plant— about 5 percent by mass—not much, right? Also, when you smoke tobacco, most of the nicotine burns away before you can even inhale it. That means that most of the nicotine in your cigarettes never gets to your lungs. It’s not ingested. So between those two things—the low proportion of nicotine in the tobacco plant, and the high proportion of the nicotine that’s destroyed before you ingest it—you’re not really getting a whole lot with each puff. Even so, the nicotine in cigarettes has a powerful effect on the body for a few reasons. For one thing, while some drugs pass through the body largely without being absorbed, nicotine inhaled into the lungs is absorbed almost completely. This is because the lungs have a lot of surface area that’s usually used to exchange oxygen and carbon dioxide when you breathe. All that surface area provides lots and lots of places for nicotine to pass into the bloodstream. Then there’s the fact that nicotine crosses the blood-brain barrier quickly. The blood-brain barrier is a sort of gatekeeping system in the bloodstream. It severely limits the ability of substances and organisms to pass from the blood into the brain. This helps keep the chemical environment of the brain stable, which it needs to function properly, and it also keeps the brain clean...you know, nasty infections can’t get in, even if the rest of the body is sick fighting them off. Because nicotine is one of those chemicals that crosses the blood-brain barrier quickly, it affects the nervous system right from its command center. And it has two big effects. One is that it stimulates the production of the hormone adrenaline. More adrenaline means an increase in your heartbeat, your breathing, and possibly even cognition—that is, the thinking and perceiving nerves in your brain. That would help... right?...to explain why people report feeling energized and sharper, more aware, after smoking a cigarette. Their breathing and heart rates are jazzed up, and their thinking nerves are too. You may be thinking at this point, Well, that’s how people get addicted to smoking—it makes them feel more energetic and stuff, and you’re partially right, but there’s another physiological reason. Scientists have evidence that nicotine stimulates the production of not only adrenaline but also another chemical: dopamine. Levels of dopamine rise when you’re doing something enjoyable, like eating your favorite food. So what happens is...some researchers refer to it as a reinforcement or reward mechanism. Dopamine signals well-being, so when nicotine stimulates dopamine production, it’s kind of like exciting your pleasure center. That’s probably the source of the rush people associate with smoking, and why they keep reaching for more cigarettes to get that same feeling again after it passes.  Now, those are the most obvious direct affects, but nicotine alters body functions in other ways that are also significant. For instance, it decreases the amount of oxygen that gets to the brain, which can make you feel more relaxed. It also, at least in the short term, relaxes the stomach. In fact—some of you may know this, but—nicotine used to be prescribed as medicine for these reasons. ... Correct Answers: 1. C 2. C,D 3. D 4. A 5. B 6. B,C
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