【单选题】

Strange things happen 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 theAtlantic Ocean, your ship enters a different time zone every day,As you enter each zone, the time changes one hour. Traveling west, you set clock back; traveling 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 InternationalDate Line.By agreement, this is the point where a new day begins. When you cross the line, you change your calendar one full day, backward or forwarD、Traveling east, today becomes yesterday; traveling west, it is tomorrow.
Strange things happen to time when you travel because ______.
A．no day really has twenty-four hours
B．the earth is divided into time zones
C．time zones are not all the same size
D．no one knows where time zones begin