The alarm clock that determines the activity of ce

单项选择题

Questions 41 to 44 are based on the following passage: Scientists now believe that many, if not all, living things are born with some type of hidden clock. Theseclocks are sometimes set by the number of hours of light or darkness in a day, by the rhythm of the tides orby the seasons. One of the most remarkable of nature’s living clocks belongs to the fiddler crab, that familiar beachdwellerwith tile overgrown claw. Biologists have long known that the crab’s shell is darkest during the day,grows pale in late afternoon, then begins to darken again at daybreak. This daytime darkening is valuable forprotection against enemies and sunlight, and for many years it was thought to be a simple response by thecrab to the sun--just as if we were to get a tan during the day and lose it at night. But when an enterprising scientist placed a fiddler crab in darkness, be was amazed to find that the colorof the crab’s shell kept ticking off the time with the same accuracy. Yet another startling fact was revealed: the crab’s shell reached the darkest color about 50 minutes lateach day. There was a second clock inside the crab, for the tides also occur 50 minutes later from day to day.Moreover, even when the crabs were taken from the beach and put back in the dark, they continued theirtidal rhythm. More research disclosed that a crab from Cape Cod, Massachusetts, reached its darkest colorfour hours earlier than the one taken from a beach on a neighboring island. The tides on the nearby islandwere found to be exactly four hours later than the Cape Cod tides. Ants don’t carry calendars around with them any more than fiddler crabs possess real wrist watches. Butants show amazing accuracy as to the day of the year. Each year, an ant nest sends out winged, young queenson mating flights. Hundreds of them may fly out of a single nest in the soil. Last summer, at the crest of mymountain, I watched an ant city prepare to send forth its young queens. At the precise moment that they tookwing, a colony of the same species that my wife was watching near the bottom of the mountain, also sent itsqueen on a wedding flight. There was, of course, no way could the two colonies have checked take off timewith each othen Entomologist Albro T. Gaul once jotted down in his notebook that a particular the same time!This split-second timing is not always the rule. However, most flights take place within a definite period oftime. Birds also have built-in timepieces which send them off on fall and spring migrations. What thebirds really have is a clock like mechanism which allows them to time hours of darkness or light in each day. But what sends birds northward again in the spring New research by Dr. Albert Wdifson ofNorthwestern University seems to indicate that the timing of return flight is extraordinarily complex. In thefall of the year the short days and long nights cause the "clocks" in migratory birds to undergo a kind of"winding" in preparation for their spring return and breeding. Then during the late fall and winter as theclock "ticks", certain physiological changes occur in the bird. The length of each day during the winterdetermines how fast the clock will run, and hence when the "alarm" will ring for the spring migration. Theclock continues to run through breeding time, then stops--to be re-wound again the next fall.

The alarm clock that determines the activity of certain living things is governedby ____.

A．hours of daylight
B．the time of day in their native environments
C．the moon
D．something we don’t understand completely