题目内容

WATER
The second most important constituent of the biosphere is
liquid water. This can only exist in a very narrow range of tem-
peratures, since water freezes at 0℃ and boils at 100℃. This is
only a tiny range compared with the low temperatures of some
other planets and the hot interior of the earth, let the tempera- 【M1】
ture of the sun.
As we know, life would only be possible on the face of a 【M2】
planet had temperatures somewhere within this range. 【M3】
The earth's supply of water probably remains quite fairly con- 【M4】
stant in quantity. A certain number of hydrogen atoms, which
are one of the main constituents of water, are lost by escaping
from the atmosphere to out space, but they are probably just 【M5】
about replaced by new water rising away from the depths of the 【M6】
earth during volcanic action. The total quantity of water is not
known, and it is about enough to cover the surface of the globe 【M7】
to a depth of about two and three-quarter ms. Most of it - 97%
is in the form. of the salt waters of the oceans. The rest is
fresh, but three quarter of this is in the form. of ice at the Poles 【M8】
and on mountains, and cannot be used by living systems when 【M9】
melted. Of the remaining fraction, which is somewhat fewer 【M10】
than 1% of the whole, there is 10 20 times as much stored as un-
derground water as is actually on the surface. There is also a mi-
nor, but extremely important, fraction of the water supply
which is present as water vapour in the atmosphere.
【M1】

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In a provocative look at the impact of sedentary behavior. on health, a new study links time watching television to an increased risk of death. One of the most surprising findings is that it isn't just couch potatoes who were affected—even for people who exercised regularly, the risk of death went up the longer they were in front of the TV. The problem was the prolonged periods of time spent sitting still.
Australian researchers who tracked 8,800 people for an average of six years found that those who said they watched TV for more than four hours a day were 46% more likely to die of any cause and 80% more likely to die of cardiovascular disease than people who reported spending less than two hours a day in front of the tube.
Time spent in front of TVs and computers and videogames has come under fire in studies in recent years for contributing to an epidemic of obesity in the U. S. and around the world. But typically the resulting public-health message urges children and adults to put down the Xbox controller and remote and get on a treadmill or a soccer field. The Australian study offers a different take. "It's not the sweaty type of exercise we're losing," says David Dunstan, a researcher at Baker IDI Heart and Diabetes Institute, Melbourne, who led the study. "It's the incidental moving around, walking around, standing up and utilizing muscles that doesn't happen when we're plunked on a couch in front of a television." Indeed, participants in the study reported getting between 30 and 45 minutes of exercise a day, on average.
The results are supported by an emerging field of research that shows how prolonged periods of inactivity can affect the body's processing of fats and other substances that contribute to heart risk. And they suggest that people can help mitigate such risk simply by avoiding extended periods of sitting. The report, being published Tuesday in the American Heart Association journal Circulation, focuses on TV watching in part because it is the predominant leisure-time activity in many countries, researchers said, especially in the U. S. A study by ratings firm Nielsen Co. found that Americans averaged 151 hours of TV viewing a month in the fourth quarter of 2008—more than five hours a day. Dr. Hamilton says studies suggest that after just one day of inactivity, levels of HDL, or good cholesterol, which helps transport LDL or bad cholesterol out of the blood stream, can fall by as much as 20%.
Keeping such processes working more effectively doesn't require constant intense exercise, but consciously adding more routine movement to your life might help, doctors say. "Just standing is better than sitting," says Gerard Fletcher, a cardiologist at Mayo Clinic, Jacksonville, Fla., who works standing up at his computer. "When you stand up, you shuffle around a little bit and use muscles not required when you're sitting or lying down". Simple strategies for increasing activity include incorporating household chores such as folding laundry into TV-watching time or getting up to change a TV channel rather than using a remote control.
According to the study, even for people who exercised regularly, the risk of death went up the longer they were in front of the TV. What's the reason？

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