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　　A Biological Clock

　　Every living thing has what scientists call a biological clock that controls behavior. The biological clock tells ___C__1___ when to form flowers and when the flowers should open1. It tells ___A__2____ when to leave the protective cocoon and fly away, and it tells animals and human beings when to eat, sleep and wake.

　　Events outside the plant and animal ___B__3_____ the actions of some biological clocks. Scientists recently found, for example, that a tiny animal changes the color of its fur __C__4_____ the number of hours of daylight. In the short ____5__B_ of winter, its fur becomes white. The fur becomes gray brown in color in the longer hours of daylight summer.

　　Inner signals control other biological clocks. German scientists found that some kind of internal clock seems to order birds to begin their long migration ___A__6____ twice each year. Birds ____A_7_____ flying become restless when it is time for the trip, _B___8___ they become calm again when the time of the flight has ended.

　　Scientists say they are beginning to learn which __B__9____ of the brain contain biological clocks. An American researcher, Martin Moorhead, said a small group of cells near the front of the brain __D__10____ to control the timing of some of our actions. These ___C_11____ tell a person when to ___A__12_____, when to sleep and when to seek food. Scientists say there probably are other biological clock cells that control other body activities.

　　Dr. Moorhead is studying __A___13____ our biological clocks affect the way we do our work2. For example, most of us have great difficulty if we must often change to different work hours. ___B__14____ can take3 many days for a human body to accept the major change in work hours. Dr. Moorhead said industrial officials should have a better understanding of biological clocks and how they affect workers. He said __C__15____ understanding could cut sickness and accidents at work and would help increase a factory’s production.

　　1. A) scientists B) humans C) plants D) animals

　　2. A) insects B) birds C) fish D) snakes

　　3. A) effect B) affect C) effected D) affected

　　4. A) because B) for the reason that C) because of D) since

　　5. A) months B) days C) minutes D) weeks

　　6. A) flight B) fly C) movement D) transportation

　　7. A) prevented from B) ordered by C) helped by D) intruded on

　　8. A) and B) but C) therefore D) however

　　9. A) portions B) parts C) sections D) kinds

　　10. A) try B) tries C) seem D) seems

　　11. A) things B) parts C) cells D) actions

　　12. A) awaken B) wake C) awakening D) wake-up

　　13. A) how B) why C) where D) what

　　14. A) We B) It C) They D) You

　　15. A) so B) with C) such D) if