Most people who travel long distances complain of jetlag (喷气式飞行时差反应). Jetlag makes business travelers less productive and more prone 【B1】 mistakes. It is actually caused by 【B2】 of your "body clock"-a small cluster of brain cells that controls the timing of biological 【B3】 . The body clock is designed for a 【B4】 rhythm of daylight and darkness, so that it is thrown out of balance when it 【B5】 daylight and darkness at the "wrong" times in a new time zone. The 【B6】 of jetlag often persist for days 【B7】 the internal body clock slowly adjusts to the new time zone. Now a new anti-jetlag system is 【B8】 that is based on proven 【B9】 pioneering scientific research. Dr. Martin Moore-Ede has 【B10】 a practical strategy to adjust the body clock much sooner to the new time zone 【B11】 controlled exposure to bright light. The time zone shift is easy to accomplish and eliminates 【B12】 of the discomfort of jetlag. A successful time zone shift depends on knowing the exact times to either 【B13】 or avoid bright light. Exposure to light at the wrong time can actually make jetlag worse. The proper schedule 【B14】 light exposure depends a great deal on 【B15】 travel plans. Data on a specific flight itinerary(旅行路线) and the individual’’s sleep 【B16】 are used to produce a Trip Guide with 【B17】 on exactly when to be exposed to bright light. When the Trip Guide calls 【B18】 bright light you should spend time outdoors if possible. If it is dark outside, or the weather is bad, 【B19】 you are on an aeroplane, you can use a special light device to provide the necessary light 【B20】 for a range of activities such as reading, watching TV or working.

A．or
B．and
C．but
D．while