A、They built a much lighter rocket. B、They built a much faster rocket. C、They loaded the rocket with enough fuel. D、They built a

Since the beginning of history, people on the earth have always watched the sky and wondered about the things they saw. As centuries of discoveries flew by, it became obvious that humans would not be content to just gaze at the solar system. We wanted to fly into it and explore it as well.
    Once again, technology allowed us to accomplish our goal. (33)On October 4, 1957. a Soviet rocket sent the first man-made satellite into space. It was called Sputnik, the Russian word for "traveler". Even though Sputnik was nothing more than a small aluminum ball with a radio inside, it started a whole new era of space exploration.
    In May 1961 President John F. Kennedy challenged American scientists to land a person on the moon before the end of the 1960s. (34)In the quest to do this, scientists faced many obstacles. For example, to get a spaceship all the way to the moon and back would demand a rocket engine powerful enough to make the trip. (35)Scientists realized that it would be difficult for one heavy rocket loaded with fuel to reach the moon. Instead, they decided to use a multistage rocket. When the fuel in one stage of the multistage rocket is used up, that stage will fall away. This makes the rocket lighter, and the engine of the next stage can go on providing power and thrust the rocket even faster and higher. As you know, Americans successfully landed on the moon in July 1969. Since then our knowledge of the solar system has increased dramatically.