单项选择题

Without transportation, there could be no trade. Without trade, there could be no towns and cities. Towns and cities are traditionally the 1 of civilization. Therefore, transportation helps make civilization 2 .
Throughout most of the prehistoric period, people lived by hunting, fishing, and 3 wild plants. They had no beasts of burden, wheeled vehicles, or roads. People traveled 4 and carried their infants and belongings strapped to their backs or heads. Loads too heavy for one person to carry were strapped to a pole and carried by two people. 5 , people learned that they could drag loads along the ground on sledges. During late prehistoric times, people began to build sledges with runners.
When people developed agriculture and began to establish 6 settlements, trade between settlements started to develop, which created a need for better 7 of transportation. The donkey and the ox, which had been tamed for food products and farm work, helped 8 this need. The use of donkeys and oxen as beasts of burden enabled people to 9 heavier loads than they could before.
People also began to develop water transportation during prehistoric times. They built rafts of such 10 as logs or reeds. Later, people learned 11 to make dugouts and canoes. People paddled these early craft with their hands or propelled them with paddles or 12 .
The wheel was invented about 3500 B. C. and sailboat about 3200 B. C. Wheeled vehicles and sailing vessels 13 transportation. But the speed of transportation improved only 14 over the centuries.
Inventors produced the first engine 15 vehicles during the late 1700’s and the early 1800’s. This development marked the beginning of a revolution in transportation that has continued to the present.