There have been many great (21) . The first great invention was one that is still very important to day--the wheel. This made it easier to carry (22) things and to travel long distances.
In the early 1800s the world (23) 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 belier.
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 part (24) our life today.
The first part of the 20th century saw more great inventions: the helicopter in 1909, movies with sound in 1926, the computer in 1928, and jet planes in 1930. This was also a time (25) a new material was first made. Nylon came out in 1935. It changed the kind of clothes people wore.
The middle part of the 20th century brought new ways to help people (26) disea
A. in
B. for
C. to
D. of
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