单选题
At the beginning of the century, medical scientists made a surprising discovery: that we are (1) not just of flesh and blood but also of time. They were able to (2) that we all have an internal "body clock" which (3) the rise and fall of our body energies, making us different from one day to the (5) . These forces became known as biorhythms: they create the (5) in our everyday life. The (6) of an internal "body clock" should not be too surprising, (7) the lives of most living things are dominated by the 24-hour night-and-day cycle. The most obvious (8) of this cycle is the (9) we feel tired and fall asleep at night and become awake and (10) during the day. (11) the 24-hour rhythm is interrupted, most people experience unpleasant side effects. (12) , international aeroplane travelers often experience "jet lag" when traveling across time (13) . People who are not used to (14) work can find that lack of sleep affects their work performance. (15) the daily rhythm of sleeping and waking, we also have other rhythms which (16) .longer than one day and which influence wide areas of our lives. Most of us would agree that we feel good on (17) days and net so good on others. Sometimes we are (18) fingers and thumbs but on other days we have excellent coordination. There are times when we appear to be accident-prone, or when our temper seems to be on a short fuse. Isn't it also strange (19) ideas seem to flow on some days but at other times are (20) nonexistent? Musicians, painters and writers often talk about "dry spells".