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[单选题]
【单选题】NASA是什么？（）
A.美国国家航天局
B.美国国家航空航天局
C.美国国家航空局
D.美国航空航天局
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题目内容
（请给出正确答案）
A.美国国家航天局
B.美国国家航空航天局
C.美国国家航空局
D.美国航空航天局
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A.would be ideal
B.would have been ideal
C.will be ideal
D.will have been ideal
利用AIRFARERAW中的1997年数据。
(i)美国航线机票的一个简单需求函数为
其中，passen是日均乘客量，fare是平均票价，dist是航线距离(以英里计)。如果这真的是一个需求函数，a1的符号应该如何？
(ii)用OLS估计第(i)部分中的方程。估计的价格弹性是什么？
(iii)考虑变量concen，它度量了市场集中程度。(具体而言，它就是最大运家所占的商业份额。)用语言解释，要在需求方程中把concen看成外生变量，我们必须假定什么？
(iv)现在假定concen在需求方程中是外生的。估计log(fare)的约简型，并证实concen对log(fare)有正的(偏)效应。
(v)用Ⅳ估计需求函数。现在，估计的需求价格弹性是什么？它与OLS估计值相比如何？
(vi)利用Ⅳ估计值，描述座位需求如何取决于航线距离。
The Solo Trek had a 120 horsepower engine with twin fans. Only one person flies. As you fly above the roofs, you lean a little forward. You can see everything under you. You are flying like Superman.
Michael Moshier looked at the jet belt and the rocket belt that was developed 20, 30 years ago. Nothing ever came from them. People still can't fly.
Inventors have tried to make it easy for people to fly. Paul Moiler has been working on his flying car for 30 years. He now says it is ready for tests. It would take off and land vertically, go 600 miles an hour, and deliver 20 miles to the gallon. A computer would do the actual flying. He says it could be sold next year for about a million dollars.
NASA is working with Moshier to help develop his flying machine. The first users are likely to be military.
It's been 50-years since Robert Fulton invented his airphibian, a flying car. It flew, and is now in the Smithsonian Museum.
Getting dreams to fly is never easy.
The Solo Flyer is able to lift off the ground by using ______.
A．a solar powered engine
B．engine-powered twin fans
C．large flapping wings
D．rotating blades
