五、完形大练兵
Look at your computer keyboard. Have you ever wondered ____21____ the letters are not in alphabetical(字母的) order?
To answer this question, let’s turn the clock back. About 150 years ago, people wrote all letters and papers by hand. That was ____22____. Later, some inventors produced typing machines. The first typewriters(打字机) were big and heavy. They worked ____23____ a piano.
Have you ever seen the inside of a piano? You press a key and some levers(杠杆) will make a hammer(音槌) hit the right string(弦) to ____24____ a sound. Early typewriters were similar. They had many levers. At the end of each lever, there was a letter. One ____25____ the A key and the A lever would hit the paper and type A. The paper then ____26____ a bit to the left. One pressed more keys and he could type a word or even a book.
The first typewriters had the ____27____ in alphabetical order. There was a problem: If one pressed two keys quickly, the levers would ____28____ become unable to move.
To ____29____ this problem, people tried to change the positions of the letters. An American inventor invented the QWERTY keyboard. It turned out to be a great ____30____. People around the world began to learn to use the keyboard.

21.B 22.A 23.C 24.D 25.A 26.B 27.D 28.D 29.A 30.A