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城镇最低收人家庭人均廉价住房保障面积原则上不超过当地人均住房面积的50%。()
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It is in the joints of human body that movements of the bones take place. The movement itself is caused by the pull of sheets and cords of very tough tissue(组织) called muscle. Muscle tissue has the special ability to shorten itself so that the bone on which it pulls has to move. When muscle tissue shortens, it also bunches up (凸起). Muscle tissue covers the body in sheets, and bands that lie between the skin and the skeleton. The bones are the framework of the body, but the muscles fill out the body shape. Most muscles extend from one bone to another. When the muscle between the bones shortens, one bone has to move. The point where the muscle is fastened to the unmoving bone is called the origin of the muscle, whereas the point where the muscle is fastened to the bone that is to be moved is called the insertion. Sometimes the muscle is not attached directly to the bone but to a tough, non-stretchable cord, or tendon that is attached to the bone.
Muscles do not push; they can only pull. To bend the arm at the elbow the muscle at the front of the upper arm has to shorten and bunch up. To unbend the arm, other muscles in the back of the arm have to shorten. These two sets of muscles--the front and back--are said to act in opposition to each other. When one set is working, the other set is usually relaxed. But there are times when both of them work. Sometimes muscles are called upon to do more than simply pull in one direction. They may have to perform. a turning motion. To be able to do this, the muscle must be attached to the bone at an angle. By pulling, the muscle can cause the bone to pivot.
According to the passage, movement occurs when a muscle______.
酶活性测定的反应体系的叙述，正确的是
Earth: Melting in the Heat？
Glaciers are melting; the ice caps are disappearing into the oceans; sea levels may rise by many meters as a consequence. Indigenous(本土的) Arctic peoples will find their food stocks gone, while flesh water supplies in Asia and south America will disappear as the glaciers which provide them melt away; penguins, polar bears and seals will find their habitats gone, their traditional lives unlivable.
But how realistic is this picture？ Is the world's ice really disappearing, or is it unscientific hot air？
A European satellite named Cryosat was designed to provide definitive answers to some of these questions. A launcher fault destroyed the mission in October 2005, hut the European Space Agency has approved a replacement, in the meantime, here is our global snapshot.
The Antarctic
Huge, pristine(质朴的), dramatic, unforgiving--the Antarctic is where the biggest of all global changes could begin.
There is so much ice here that if it all melted, sea levels globally would rise hugely-perhaps as much as 80m. Say goodbye to London, New York, Sydney, Bangkok...in fact, the majority of the world's major cities.
But will it happen？ Scientists divide the Antarctic into three zones: the east and west Antarctic ice sheets; and the Peninsula, the tongue of land which points up towards the southern tip of South America.
"Everybody thinks that the Antarctic is shrinking due to climate change, but the reality is much more complex," says David Vaughan, a principal investigator at the British Antarctic Survey in Cambridge, U.K. "Parts of it appear to be thickening as a result of snowfall increases, but the Peninsula is thinning at an alarming rate due to warming. The West Antarctic sheet is also thinning, and we're not sure of the reason why."
On the Up
Temperatures in the Peninsula appear to be increasing at around twice the global average—about 2℃ over the last 50 years. Those figures are based on measurements made by instruments at scientific stations.
Earlier this year, David Vaughan's group published research showing that the vast majority of glaciers along the Peninsula--87% of the 244 studied--are in retreat. The ice dumped into the ocean as the glaciers retreat should not make much difference to global sea levels--perhaps a few centimeters.
More worrying, potentially, are the vast ice sheets covering the rest of Antarctica. Making temperature measurements for the continent as a whole is difficult; it is a vast place--more than 2,000 km across---there are few research stations, and temperatures vary naturally by 2-3 ℃ from year to year. But measurements indicate that in the west, melting is underway.
"About one-third of the West Antarctic ice sheet is thinning," says Dr. Vaughan, "un average by about 10 cm per year, but in the worst places by 3～4m per year."
The rock on which the West Antarctic ice rests is below sea level, and British Antarctic Survey researchers believe the thinning could be due to the ice sheet melting on its underside.
"It may be that the ocean is warming end that's causing the ice to melt, but there may be other reasons as well; for example, there's lots of volcanism in that area and so that could change how much heat is delivered to the underside of the ice sheet."
Cryosat should help to pin down what is happening at the West Antarctic fringe. The radar altimeters on board its predecessors ERS1 and ERS2 have been unable to map the steep slopes at the coast, whereas Cryosat's instrument should be able to cope.
If the entire West Antarctic ice sheet did melt, sea levels globally would rise, by around 5m. But at the moment, there is no sign of that happening.
One recant scientific paper attempted to calculate probabilities for how much West An
The statement "theories are facts" may be called ______.
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