35.If the construction workers had lived in the late nineteenth century,they would probably have A.built railroads B.raised cattle C.planted crops Part 3.Questions 36-40 are based on this part.(20 points,4 points each) Travel in space Travel in space presented a big problem.How could an object get away from the pull of the earth?What was needed was a special engine.It would have to produce enough force to lift a spacecraft weighing many tons.Airplane engines use oxygen.There is no oxygen in space.So a spacecraft needs an engine that works without oxygen.Finding the right kind of engine took many years.Modern scientists used a very old idea-the rocket. A rocket is an engine that burns fuel and moves forward by pushing exhaust backward. Rockets operate by burning fuel in a tube.When the fuel burns,the hot gases produced are allowed to escape in one direction only.This causes a force to push in the opposite direction. Rockets do not need the presence of air.Any oxygen they need is in the fuel mixture that they burn. Rockets can be used to place satellites in orbit.The rocket provides the power for the satellite to get into space.Once the satellite is far enough away from the earth,the rocket falls off.The satellite then circles around the earth on its own. In 1957,the Soviet Union used a rocket to put the first spacecraft into orbit around the earth.It was called Sputnik I.One year later,the United States sent a spacecraft named Explorer I into space.These events marked the start of the space age.Since then,powerful rockets have placed heavier satellites into orbit.They have carried astronauts to the moon. Rockets have launched space probes to fly near the other planets.Powerful computers help these spacecraft to stay on course. Large spacecraft are very expensive to build.The United States has developed a spacecraft that is powerful and can be reused.It is the space shuttle.The space shuttle is different from other spacecraft.It can do three important things.It takes off like a rocket.It can fly in orbit around the earth like a rocket.Most important of all,it can land like an airplane.The shuttle is used to place satellites in orbit.It can also repair satellites that are in orbit.Someday the shuttle will carry people to orbiting space stations.Best of all,the same 72B. raised cattle 35. If the construction workers had lived in the late nineteenth century. they would probably have A. built railroads C. planted crops Part 3. Qu臼lions 36 - 40 are based on this part. (20 points, 4 points each) Travel in space Travel in space presented a big problem. How could an object get away from the pull of the earth? What was needed was a special engine. It would have to produce enough force to lift a spacecraft weighing many tons. Airplane engines use oxygen. There is no oxygen in space. So a spacecraft needs an engine that works without oxygen. Finding the right kind of engine took many years. Modern scientists used a very old idea-the rocket. A rocket is an engine that burns fuel and moves forward by pushing exhaust backward. Rockets operate by burning fuel in a tube. When the fuel burns. the hot gases produced are allowed to escape in one direction only. This causes a force to push in the opposite direction. Rockets do not need the presence of air. Any oxygen they need is in the fuel mixture that they burn. Rockets can be used to place satellites in orbit. The rocket provides the power for the satellite to get into space. Once the satellite is far enough away from the earth. the rocket falls off. The satellite then circles around the earth on its own. In 1957 , the Soviet Union used a rocket to put the first spacecraft into orbit around the earth. It was called Sputnik I. One year later, the United States sent a spacecraft named Explorer I into space. These events marked the start of the space age. Since then , powerful rockets have placed heavier satellites into orbit. They have carried astronauts to the moon. Rockets have launched space probes to fly near the other planets. Powerful computers help these spacecraft to stay on course. Large spacecraft are very expensive to build. The United States has developed a spacecraft that is powerful and can be reused. It is the space shuttle. The space shuttle is different from other spacecraft. It can do three important things. It takes off like a rocket. It can fly in orbit around the earth like a rocket. Most important of all. it can land like an airplane. The shuttle is used to place satellites in orbit. It can also repair satellites that are in orbit. Someday the shuttle will carry people to orbiting space stations. Best of all , the same 72