In this high-tech digital world, it is very convenient for us to send messages. But in ancient times, people had to think hard to find ways to get the message sent.
Roman soldiers in some places built long rows of signal towers. When they had a message to send, the soldiers shouted it from tower to tower. If there were enough towers and enough soldiers with loud voices, important news could be sent quickly over distance.
In Africa, people learned to send messages by beating on a series of large drums. Each drum was kept within hearing distance of the next one. The drum beats were sent out in a special way that all the drummers understood. Though the messages were simple, they could be sent at great speed for hundreds of miles.
In the eighteenth century, a French engineer found a new way to send short messages. In this way, a person held a flag in each hand and the arms were moved to various positions representing different letters of the alphabet. It was l
A. The amount of weapons.
B. The tower height.
C. The weather condition.
D. The voice volume.
| The average population density of the
world is 47 persons per square mile. Continental densities range from no
permanent inhabitants in Antarctica to 211 per square mile in Europe. In the
western hemisphere, population densities range from about 4 per square mile in
Canada to 675 per square mile in Puerto Rico. In Europe the range is from 4 per
square mile in Iceland to 831 per square mile in the Netherlands. within
countries there are wide variations of population densities. For example, in
Egypt, the average is 55 persons per square mile, but 1,300 persons inhabit each
square mile in settled portions where the land is arable. High population densities generally occur in regions of developed industrialization, such as the Netherlands, Belgium, and Great Britain, or where lands are intensively used for agriculture, as in Puerto Ri A. 1,300 persons B. few inhabitants C. pyramids D. many settlements [简答题]{{B}} AVERAGE FAMILY EXPENDITURE IN AMERICA (%){{/B}}
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