单选题
Men have traveled ever since they first appeared on the earth. In primitive times they did not travel for pleasure but to find new places where their herds could feed, or to escape from hostile neighbors, or to find more favorable climates. They traveled on foot. Their journeys were long, tiring, and often dangerous. They protected themselves with simple weapons, such as wooden sticks or stone clubs, and by lighting fires at night and, above all, by keeping together. Being intelligent and creative, they soon discovered easier ways of traveling. They rode on the backs of their domesticated animals; they hollowed out tree trunks and, by using bits of wood as paddles, were able to travel across water. Later they traveled, not from necessity, but for the joy and excitement of seeing and experiencing new things. This is still the main reason why we travel today. Traveling, of course, has now become a highly organized business. There are cars and splendid roads, express trains, huge ships and jet airliners, all of which provide us with comforts and security. This sounds wonderful. But there are difficulties. If you want to go abroad, you need a passport and a visa, ticket, luggage, and a hundred of other things. If you lose any of them, your journey may be ruined.