Sequence. Number the topics from 1 to 5 in the order they are discussed in the video. A: how spacesuits protect astronauts B: how spacewalkers train before going to space C: the first two people who performed spacewalks D: why spacewalking is difficult work E: tasks that spacewalkers perform

A)It was 45 years ago that Neil Armstrong took the small step onto the surface of the moon that changed the course of history. The years that followed saw a Space Age of scientific, technological and human research, on which we have built the modern era. Starting from the moon, we will move deeper into the solar system. The Apollo missions cleared a path for human exploration to the moon. Today we are extending that path to near-Earth asteroids(小行星), Mars and beyond. B)We're building on the Apollo program's accomplishments to test and fly novel(新的), cutting-edge(尖端) technologies today for tomorrow's missions. As we develop and test the new tools of 21st century spaceflight on the journey to Mars, we once again will change the course of history. C)The path to Mars begins with research on Earth and extends beyond its bounds, aboard the orbiting laboratory of the International Space Station, with our international partners. Some 250 miles above our heads, astronauts are conducting hundreds of experiments not possible on Earth, teaching us how humans can live, work and thrive(长得健壮) for longer periods in space. D)To help this nation send humans to deep space and return them to Earth safely, engineers across the country are developing a new space transportation tool that can travel far beyond our home planet. The Orion spacecraft and Space Launch System (SLS) heavy-lift rocket will be the most advanced space vehicles ever built. Together, they will take us farther into the solar system than humans have ever traveled. They are our spaceship to Mars and beyond. E)As we turn our eyes toward Mars, we are designing missions to take us to a "proving ground"around the moon called cis-lunar(月球周围) space, where some of the very building blocks of the solar system can be explored. F)Near-Earth asteroids provide a unique opportunity we need for future human missions to Mars. Around 2019, we'll launch a robotic mission to reach a near-Earth asteroid. The spacecraft either will capture an entire asteroid or take a part off a much larger asteroid, then redirect the asteroid mass to a stable orbit(轨道) around the moon. Such missions will provide important information about the space to safely send humans to Mars. G)The new technologies we test by sending astronauts to study the asteroids in the cis-lunar space will make important advances to safely send humans to Mars. This includes tools like Solar Electric Propulsion, a highly efficient way to help us transport large objects and heavy cargo to support future Mars missions. NASA will continue to make significant investments in new technologies vital to achieving exploration goals. This includes advancements in entry, descent and landing technologies such as Low Density Supersonic Decelerators. H)Sending humans to deep space around the moon also will help advance techniques for space operations on and around Mars and its moons. The space around our moon is different than low-Earth orbit but very similar to what a spacecraft will experience on the trip to and from Mars. For instance, solar and cosmic radiation is intense. We also can use cis-lunar space to begin practicing activities in deep space, like spacewalks, and learn to cope with delays in communication with Earth because of the distance. I)Mars calls for us to explore. Missions to Mars could answer some of the fundamental questions of humanity: Does life exist beyond Earth? Could humans live on Mars in the future? J)The journey to answer these questions has risks, but the rewards for humanity are worth it. In our lifetime, NASA and the world will take the next giant leap to explore the red planet. ______11.Preparation for the journey to Mars can start from a place near Earth. ______12.People from different countries contribute to the exploration of Mars in one way or another. ______13.Great benefits will be gained from missions to Mars. ______14.A number of new technologies will be needed to send human beings to Mars. ______15Despite the risks, it is worthwhile to take this great leap to explore Mars. ______16.We are now standing on a new horizon, ready to take the next giant leap. ______17.If we want to travel to Mars, a more powerful form of transportation is needed. ______18.The cis-lunar space will serve as a genuine training ground. ______19.New missions near Earth will provide new information before we really send a human being onto Mars. ______20.The Apollo program has provided a strong base for our future explorations.