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厂商实现生产要素最优投人的条件是劳动的边际产量等于资本的边际产量。（）
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从（）起北美自由贸易区开始运转。
SECTION C NEWS BROADCAST
Directions: In this section you will hear everything ONCE ONLY. Listen carefully and then answer the questions that follow. At the end of each news item, you will be given 10 seconds to answer the questions.
听力原文： The first prototype of an aircraft intended to revolutionize travel and shatter speed records was destroyed during its maiden flight on Saturday after a booster rocket carrying it a loft veered out of control and tumbled from the sky.
US space scientists were forced to destroy tile un- manned X-43A prototype in midair just minutes before the plane was due to fire the oxygen scooping "scramjet" engine designed to drive it at seven times the speed of sound.
NASA cameras aboard two F-18 chase planes showed the Pegasus rocket careening off course and fulling out of control from the sky before controlers triggered on board explosures to destroy it over the Pacific Ocean at about 1:45 pm PDT (4:45 pm EDT/2045 GMT)
Pieces of the booster rocket and the small, black X- 43A prototype, part of NASA’s "Hyper-X" programme, fell into the sea.
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