A、Gas and dust. B、Gas and water. C、Dust and water. D、Gas and remains. A选项都是名词。题目问的是太阳是由什么构成的。文中提到气体和灰尘构成了太阳、地球，以及生命。由此可知，正确答案是A)

Our Milky Way solar system began as small pieces of star-created gas and dust. They came together to form the Sun, the Earth and other planets, and life as we know it. But not all the dust was used, and what remains can be dangerous. Scientists are studying this dust with a student-designed instrument on the American space agency’s New Horizons spacecraft. The agency is busy collecting information from the spacecraft this week in the first-ever flyby of Pluto.
David James is a research assistant at the University of Colorado. Our reporter found him in one of the university’s laboratories on a recent day. He was inspecting the settings on a room-size machine.
    ’’That’s one of the cryopumps. You have to continuously pull air out otherwise it will leak up to the atmosphere again. ’’Those pumps take air from a tunnel or passageway. The process helps scientists make dust on Earth move as fast as it would in space. ’’We’ve even reached 107 kilometers per second is our fastest particle here. ’’The tests may help scientists learn what causes dust to be created in space. It may also help them design spacecraft and equipment that cannot be hurt by dust.
    Tiffany Finley was also a graduate student when she began working on the dust counter project, in 2002. ’’This opportunity came up and I said ’What? You’re going to Pluto? I would love to be part of that.’ ’’The dust counter was designed to measure and count the dust that hit and flew by the New Horizons spacecraft as it traveled to Pluto. Tiffany Finley says such a device would usually be created by experts. She later became the manager of the dust project. Today, Ms. Finley is the Science Operations Manager for the New Horizons mission.
    Mihaly Horanyi is a professor of physics at the University of Colorado. He also serves as the faculty advisor for the Student Dust Counter project. He says the counter did not cost as much money as it would have if a company built it because so many students were involved. More than 30 students have worked on the project.
    The Student Dust Counter was launched with the New Horizons spacecraft in 2006. Six devices on the almost 5-billion-kilometer-long journey were designed to stay quiet during the trip. Now they are operating.They are sending color pictures back to Earth, and studying Pluto’s chemical makeup. But the Student Dust Counter has been operating throughout the trip.
20. What is the sun made of?
21. What was David James doing when the reporter found him?
22. How many students have worked on the Dust Counter project?