第二节 阅读填空（每小题1分，共5分）
It is very natural that if you have two dollars and add two dollars to them, you have four dollars. But do you know it took man millions of years to be able to think this way? 51. ______
A
In old times, when a man wanted to tell how many animals he had, he had no system of numbers to use. He put a stone or pebble into a bag for each animal. 52. ______
D
It may explain why the word “calculate” comes from the Latin word calculus, which means “stone”.
53. ______
E
People would just draw a line or tally (记录) mark for each object they wanted to count, but they had no words to tell the numbers.
The next step in the development (发展) of the number system was probably to use fingers. 54. ______
C
The word “digit” comes from the Latin word “digitus”. It means “finger”! And the fact that we have 10 fingers led to the general use of 10 in the system of numbers.
55. ______
B
It was brought to Europe during the 8th to 11th century by Arab traders. In this system all numbers are written with the ten digits 1, 2, 3, 4, 5, 6, 7, 8, 9 and 0. It is a decimal system, that is, it is built on the base of 10.
A. In fact, one of the most difficult things to teach children is the idea of numbers.
B. The number system we use today was invented by the Hindus in India thousands of years ago.
C. Again we have a word that goes back to this.
D. The more animals, the more stones or pebbles he had.
E. Later on, man used tally marks to count.
