What is the correct name for the ionic compound, MgF2?
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Whаt is the cоrrect nаme fоr the iоnic compound, MgF2?
DIRECTIONS: Chооse the best аnswer fоr eаch question. The World's Fаvorite Drug [A] It's 1:45 a.m., and 21-year-old Thomas Murphy is burning the midnight oil, studying for an important engineering exam he has at 2:00 in the afternoon later today. To stay awake and alert, he's had two cups of coffee in the last three hours and is now downing a popular energy drink - one that has two to three times the amount of caffeine as a similar sized can of soda. Many students like Murphy, as well as marathon runners, airline pilots, and long-distance travelers, owe their energy to one of humankind's oldest stimulants: caffeine.[B] The power to counter physical fatigue and increase alertness is part of the reason caffeine ranks as the world's most popular mood-altering drug. It is found not only in sodas, energy drinks, coffee, and tea, but in diet pills, pain relievers (like aspirin), and chocolate bars. Many societies around the world have also created entire rituals around the use of caffeine. For example, there's the cafe culture of France, the tea ceremony in Japan, and the morning cup of coffee or tea that marks the start of the day in many cultures.[C] Caffeine is present in many of the foods and drinks we consume, but is it good for us? Charles Czeisler, a scientist and sleep expert at Harvard Medical School, believes that caffeine causes us to lose sleep, which he says is unhealthy. "Without adequate sleep - the typical eight hours - the human body will not function at its best, physically, mentally, or emotionally." Too often, Czeisler says, we consume caffeine to stay awake, which later makes it impossible for us to get the rest we need.[D] Health risks have also been tied to caffeine consumption. Over the years, studies have attributed higher rates of certain types of cancer and bone disease to caffeine consumption. To date, however, there is no proof that caffeine actually causes these diseases.[E] A number of scientists, including Roland Griffiths - a professor at the Johns Hopkins School of Medicine in the United States - believe that regular caffeine use causes physical dependence. Heavy caffeine users, Griffiths says, exhibit similar behaviors. For example, their moods fluctuate from high to low, they get mild to severe headaches, or they feel tired or sad when they can't have a caffeinated drink. To minimize or stop these feelings, users must consume caffeine - a behavior Griffiths says is characteristic of drug addiction.[F] Despite these concerns, the general opinion in the scientific community is that caffeine is not dangerous when consumed in moderation. This means having one or two small cups of coffee (about 300 milligrams of caffeine) per day, for example. Furthermore, a lot of current research contradicts long-held negative beliefs about caffeine, and suggests that it may, in fact, have health benefits. For instance, studies have shown that caffeine can help ease muscle pain. Because it is a stimulant, caffeine can also help improve one's mood. Research has also shown that some caffeinated drinks - specifically certain teas - have disease-fighting chemicals that can help the body fight a number of illnesses, including certain types of cancer.[G] In addition, as a type of mental stimulant, caffeine increases alertness, memory, and reaction speed. Because it fights fatigue, it facilitates performance on tasks like driving, flying, and solving simple math problems. And while it is true that caffeine can increase blood pressure, the effect is usually temporary and therefore not likely to cause heart trouble. This is especially true if caffeine is consumed in moderation. Moreover, despite its nearly universal use, caffeine has rarely been abused. "With caffeine, overuse tends to stop itself," says Jack Bergman, a specialist at Harvard Medical School. If you consume too much, "you get ... uncomfortable, and you don't want to continue."[H] Caffeine's behavioral effects are real, but most often mild. Getting that burst of energy, of course, is why many of the world's most popular drinks contain caffeine. Whether it's a student drinking coffee before class or a businessperson enjoying tea with lunch, humankind's favorite stimulant is at work every day, all over the world.What is the main idea of paragraph A?
DIRECTIONS: Chооse the best аnswer fоr eаch question. Addicted to Distrаction [A] David Strayer never texts or talks on a phone when he is driving. Strayer is a cognitive psychologist at the University of Utah who specializes in attention. His research shows that, when driving, using a phone can be as dangerous as drinking alcohol. [B] Strayer and other scientists warn against multitasking. When we do many things at the same time, Strayer says, our brain becomes tired easily. We make mistakes and lose focus. Yet multitasking is increasingly common, especially on a phone or computer. Dr. Gloria Mark, a digital media researcher, has found that people in the United States switch tasks on their computers 566 times a day. This type of multitasking is especially common among teenagers. On average, a teenager spends between 7 and 11 hours a day in front of a screen - doing homework, texting, watching videos - often all at the same time.[C] Phones, tablets, and laptops are all useful tools, and we may feel more productive when we use them. However, when we spend hours on digital devices - constantly switching between activities - our overall productivity decreases. Why?[D] Many digital activities require high levels of concentration. When we text or email someone, for example, or even play games, we have to focus. When we do this, our brain uses energy. When we multitask - which is hard for the brain to do - we use even more energy. Excessive multitasking reduces our ability to think clearly and creatively, and to remember information. This, in turn, can result in stress. [E] What is the best cure for our overstressed minds? David Strayer says the answer is simple: Spend some time in nature.A Natural Remedy[F] Strayer and other scientists are studying nature's effect on our brains and bodies. When we are in natural environments, the prefrontal cortex (the brain's control center) relaxes. Studies show that when people can see trees and grass, they are calmer and do better in school. Indeed, people even relax when they look at photos of nature. Consequently, they do better on different cognitive tasks.[G] Other psychologists are also studying "nature therapy." In a study at Chiba University in Japan, 84 subjects went for a 15-minute walk in seven different forests. The same number of people walked around different city centers. Researchers then took blood from each person. The forest walkers had a 16 percent decrease in the stress hormone cortisol. In addition, they had a 2 percent drop in blood pressure and a 4 percent drop in heart rate. All did better than the city walkers. Lead researcher Yoshifumi Miyazaki has an explanation for these results. Our bodies relax in pleasant, natural surroundings, he says. Humans evolved in that environment, not in places with tall buildings and lots of traffic.[H] Psychologist Stephen Kaplan and his colleagues have done similar research. In one study, people took a 50-minute walk in a public garden. In a test conducted afterwards, their short-term memory improved. When the same people walked on a city street, it did not.[I] Kaplan says it is the visual details in nature - sunsets, streams, butterflies - that reduce stress and mental fatigue. We enjoy them without having to concentrate on them. This allows our brains to rest and recover from the stresses of modern life. "Imagine a therapy that was readily available, and could improve your cognitive functioning at zero cost," Kaplan says. "It exists: it's called 'interacting with nature.'"According to research, what do Americans do 566 times a day?