A、Adjusting his biological clock. B、Knowing the direction of a jet. C、Realizing the time difference during flight D、Getting used

Jet lag, caused by traveling between time zones, is becoming a common problem for frequent travelers: for 49 percent it is only a nuisance and for 45 percent it is a real problem. (16)It is caused by disruption to the internal biological clock, and may lead to digestive problems, tiredness, and sleep disruption. Generally speaking, our biological clock is slightly disturbed if we just move into the next time zone, but jet lag becomes a problem once we have passed through three or four time zones. (17)The body takes about one day to get over each hour of time difference. But the seriousness of jet lag problems also depends on our direction of travel. If we travel west we will be in advance of ourselves as far as our internal clock is concerned, and problems may arise. A west-to-east journey, on the other hand, makes us late compared to the local time. It often demands even greater effort in adjustment since we are not quick enough to catch up with the new time schedule. Therefore, a trip from New York to Los Angeles often causes fewer problems than a Los Angeles-New York trip.
16. What’s the problem a person suffers from when traveling between time zones?
17. How many hours does one need to get over his jet lag problem if he travels across three time zones?
18. What’s the main purpose of the passage?