A Biological Clock

Every living thinghas what scientists call a biological clock that controls behavior. The bio-logical clock tells______(51)when to form flowers and when the flowers should

open. It tells______(52)when to leave the protective cocoons and fly away,and it tellsanimals and human beings when to eat,sleep and wake.

Events outside theplant and anima______(53)the actions of some biological clocks.Sci- entistsrecently found,for example,that a tiny animal changes the color of its

fur______(54) thenumber of hours of daylight.In the short______(55)of winter,its fur becomeswhite.The fur becomes gray brown in color in the longer hours of daylight in

summer.

Inner signalscontrol other biological clocks.German scientists found that some kind of internalclock seems to order birds to begin their long migration______(56)twice each

year. Birds______(57)flying become restless when it is time for the trip,______(58)theybecome calm again when the time of the flight has ended.

Scientists say theyare beginning to learn which______(59)of the brain contain biologica] clocks.AnAmerican researcher,Martin Moorhead,said a small group of cells near

the front of thebrain______（60)to controlthe timing of some of our actions.These______(61)tell a person when to______(62),whento sleep and when to seek

food.Scientists saythere proba- bly are other biological clock cells that control other bodyactivities.

Dr Moorhead isstudying______(63)our biological clocks affect the way we do our work .Forexample,most of us have great difficulty if we must often change to different

work hours.

______(64)can takemany days for a human body to accept the major change in work hours .Dr. Moorhead said industrialofficials should have a better understanding of

biological clocksand how they affect workers.He said______(65)understanding could cut sicknessand accidents at work and would help increase a factory's production.58._________

A.and

B. but

C. therefore

D. however