听力原文： There have been many great inventions, things that change the way we live. The first great invention was one that is still very important today—the wheel. This made it easier to carry heavy things and to travel long distances. For hundreds of years after that there were few inventions that had as much effect as the wheel. Then in the early 1800s the world started to change. There was little unknown land left in the world. People did not have to explore much any more. They began to work instead to make life better. In the second half of the 19th century many great inventions were made. Among them were the camera, the electric light and the radio. These have all become a big part of our life today. The middle part of the 20th century brought new ways to help get over diseases. They worked very well. They made people healthier and let them live longer lives. By the 1960s most people could expect to live to be at least 60. By this time most people had a very good life. Of course new inventions continued to be made. But man now had a desire to explore again. The world was known to man, but the stars were not. Man began looking for ways to go into space. Russia made the first step. Then the United States took a step. Since then other countries, including China and Japan, have made their steps into space. When did people stop exploring a lot?