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　　In May 1989, space shuttle “Atlantis�?released in outer space the space probe “Megallan,�?which is now on her 15-month and one-billion-kilometer flight to Venus. A new phase in space exploration has begun.

　　The planet Venus is only slightly smaller than Earth; it is the only other object in the solar system, in fact, that even comes close to earth’s size. Venus has a similar density, so it is probably made of approximately the same stuff, and it has an atmosphere, complete with clouds. It is also the closest planet to earth, and thus the most similar in distance from the sun. In short, Venus seems to justify its long-held nickname of “earth’s twin.�?/p>

　　The surface temperature of Venus reaches some 900F. Added to that is an atmospheric pressure about 90 times Earth’s: High overhead in the carbon dioxide (CO2) that passes for air is a layer of clouds, perhaps 10 to 20 miles thick, whose little drops consist mostly of sulfuric acid (H2SO4). Water is all but nonexistent.

　　Born with so many fundamental similarities to earth, how did Venus get to be so radically different: It is not just an academic matter. For all its extremes, Venus is a valuable laboratory for researchers studying the weather and climate of earth. It has no earth’s oceans, so the heat transport and other mechanisms are greatly simplified. In addition, the planet Venus takes 243 earth-days to turn once on its axis, so incoming heat from the sun is added and distributed at a more leisurely, observable pace.

　　16. Venus is similar to Earth in ________.

　　[A] size and density

　　[B] distance from the sun

　　[C] having atmosphere(D)

　　[D] all of the above

　　17. The greatest value in studying Venus should be to ________.

　　[A] allow us to visit there

　　[B] understand Earth better

　　[C] find a new source of energy(B)

　　[D] promote a new space program

　　18. The main idea of this passage is about ________.

　　[A] problems of space travel

　　[B] scientific methods in space exploration

　　[C] the importance of Venus to Earth(C)

　　[D] conditions on Venus

　　�{�案�Q?6. [D]17. [B]18. [C]