题目内容
充气单位应负责妥善保管气瓶充装记录，保存时间不应少于()年。
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Snowflakes
You've probably heard that no two snowflakes are alike. Of course, nobody has ever confirmed that statement by examining every one of the estimated one septillion snowflakes that drift to Earth each year. Still, Kenneth Libbrecht, a professor at the California Institute of Technology, is confident that the statement is true.
Snowflakes aren't flaky, says Libbrecht. At their basic level, they're crystalline. The lattice of every snowflake is six-sided in shape. The simplest snow crystals are six-sided flat plates and six-sided columns. Such crystals are common in places where the air is extremely cold and dry. Snow crystals acquire their special beauty when their simple six-sided symmetry blossoms. Under the right conditions, each of the six comers of a crystal sprouts what is called an arm. In a matter of minutes, the arms can become highly ornate and give the crystal a star like appearance.
Several factors in the environment affect the shape and growth rate of a snow crystal. One factor is humidity. Crystals grow faster and in more intricate shape as humidity increases. A second factor is air temperature. A snowflake is born when several molecules of water vapor in a cloud land on a speck of dust and freeze to form. a simple crystal. As the young crystal bops around in the cloud, it passes through air pockets of varying temperatures. If the crystal passes through a pocket of air that is, say, -15 degrees Celsius, it will grow quickly and sprout six arms, says Libbrecht. If the crystal is then tossed into a warmer pocket, one about -10℃, the arms' tips will stop growing quickly and form. six-sided plates. If the crystal then drifts into an even warmer pocket of about -5℃, its top and bottom will grow more quickly than its sides and become more column like in shape.
In the course of its life span, a snow-crystal might flutter through many warmer and colder pockets, acquiring a complicated and unique growth history. Such a history will give rise to a snowflake that is unlike any other. Each arm on the snowflake will look exactly like every other one, but the crystal itself will be one of a kind.
Using his cooling tanks, Libbrecht has learned how to create snow crystals of different shapes--plates, columns, needles, etc. Libbrecht has even refined his techniques so that he can make crystals that look highly similar to one another. Still, he lacks the control to manufacture identical twin snowflakes. A slight difference in humidity and temperature can upset the growth profile6 of a crystal.
What does Professor Libbrecht believe to be true？
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