1. Strange thing happens to time when you travel. Because the earth is divided into twenty-four time zones one hour apart, you can have days with more or fewer than twenty-four hours, and weeks with more or fewer than seven days.
If you make a five-day trip across the Atlantic Ocean, your ship enters a different time zone every day. As you enter each zone, the time changes the hour. Travelling west, you set your clock back; travelling east, you set it ahead. Each day of your trip has either twenty-five or twenty-three hours.
If you travel by ship across the Pacific you cross the international date line. By agreement, this is the point where a new day begins. When you cross the line, you change your calendar the full day, backward or forward. Travelling east, today becomes yesterday; travelling west, it is tomorrow!
 
21. The best title for this selection is____.
A. A Trip Across the Atlantic
B. How Time Changes Around the World 
C. Crossing the International Date Line
D. How Time Zones Were Set Up