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, tickets, luggage, and a hundred and one other things. If you lose any of them, your journey may be ruined.

单选题 In primitive times men traveled ______.

单选题 Their journeys were long, tiring and often dangerous, so they had to take measures to protect themselves EXCEPT ______.

单选题 Later, they found it easier to travel because ______.

单选题 Traveling has now become a highly organized business ______.

单选题 "... to escape from hostile neighbors...", "hostile" means ______.