A、Progress in medicine. B、Progress in new materials. C、Progress in computers. D、Progress in helicopters. A

There have been many great inventions, things that changed the way we live. The first great invention was one that is still very important today -- the wheel. This is used in cars, trucks, and tractors, etc., and has made it easier to carry heavy things and 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 1800’s the world started to change. There was little unknown land left in the world. People did not have to explore much anymore. 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 all became a big pan of our life today.
    The first part of the 20th century saw mom great inventions: the helicopter in 1909, movies with sound in 1926, the computer in 1928, and the jet plane in 1930. This was also a time when a new kind of material was first made. Nylon came out in 1935. It changed the kind of clothes people wear.
    The middle part of the 20th century brought new ways to help people get over diseases, They worked very well. They made people healthier and let them live longer. By the 1960’s most people could expect to live to be at least 60.