题目

如图，棱长为1的正方体ABCD﹣A1B1C1D1中，M为线段A1B上的动点，则下列结论正确的有___________ ①三棱锥M﹣DCC1的体积为定值②DC1⊥D1M ③∠AMD1的最大值为90°   ④AM+MD1的最小值为2． 答案：    ①②_  Travel in the FutureThese days, if you want to travel from New York to Beijing, the flight takes about 16 hours. You also have to wait at the airport, and sometimes you have to transfer (换乘) onto a different airplane. If you add all of that extra time, the trip takes at least 20 hours. But in the future, the trip from New York to Beijing might only take two hours. Right now, a company is developing something called ET3, which stands for "evacuated tube (真空管) transport technology". Here's how it works. A tube goes all the way from New York to Beijing. The tube is only a few meters wide. Capsules (舱) move through the tube. Six people can sit in one capsule. The capsules use electricity instead of oil. For international travel, the capsules can travel at about 6,500 kilometers per hour. That's much faster than airplanes—most modern airplanes usually only fly at about 800 kilometers per hour. So how is this possible? How can these capsules travel so fast? The answer is that there is no air inside the tube. When airplanes fly, they have to move through air. The air slows the airplanes down. Because there isn't any air in the ET3 tubes, the capsules are able to move at a very high speed. Also, the capsules are quite light. Each only weighs 183 kilograms. There are other advantages to ET3. Because it uses electricity, it's quite clean. Also, it's a lot quieter than airplanes and cars. Finally, ET3 tubes and capsules don't require very much building materials. Of course, ET3 doesn't come into use yet. The technology is still being developed. Developers still have to solve a lot of problems. The biggest problem is that right now ET3 would be far too expensive. In order to make ET3 cheaper, we will need much better technology. It's exciting to think of how ET3 will change the travel in the future. Who knows? Maybe someday you'll be able to have lunch in New York and dinner in Beijing.1.According to the passage, an ET3 looks more like a ________.A.spaceship B.bus C.plane D.subway train2.ET3 travels at the very high speed because _________.A.it is powered by electricity and oil B.it is much lighter than an airplaneC.it moves through a tube without air D.it is made up of fewer capsules3.From the passage, the writer expects that ________.A.modern technology will be used in airplanes B.ET3 will take the place of airplanes some dayC.less money will be spent for passengers on ET3 D.ET3 will change the travel in the future greatly