Although scientists know that the number of hours

单项选择题

Scientists now believe that many, if not all, living things are born with some type of hidden clock. These clocks are sometimes set by the number of hours of light or darkness in a day, by the rhythm of the tides or by the seasons. One of the most remarkable of nature’s living clocks belongs to the fiddler crab, that familiar beach-dweller with the 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 for protection against enemies and sunlight, and for many years it was thought to be a simple response by the crab 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, he was amazed to find that the color of 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 later each 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 their tidal rhythm. More research disclosed that a crab from Cape Cod, Massachusetts, reached its darkest color four hours earlier than the one taken from a beach on a neighboring island. The tides on the nearby island were found to be exactly four hours later than the Cape Cod tides. Birds also have built-in timepieces which send them off on fall and spring migrations. What the birds 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 Wolfson of Northwestern University seems to indicate that the timing of remm flight is extraordinarily complex. In the fall 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 remm and breeding. Then during the late fall and winter as the clock "ticks", certain physiological changes occur in the bird. The length of each day during the winter determines how fast the clock will mn, and hence when the "alarm" will ring for the spring migration. The clock continues to run through breeding time, then stops--to be re-wound again the next fall. Scientists are now learning that many of the clocks of nature can be reset, speeded up or slowed down--all for our benefit. Pioneering experiments at the U. S. Department of Agriculture’s research center in Beltsville, Maryland, have shown that plants can be helped to develop faster in less time. By increasing or lessening the hours of darkness in each day, the scientists have been able to mm plant growth off and on like an electric switch. New knowledge about nature’s living clocks has practical applications. For man, too, seems to follow daily rhythms. The amount of sugar in our blood stream varies with the time of day, as does our temperature. More of the cells in our skin and muscles divide during the night hours than during the day. By tinkering with the clocks of plants and animals, scientists may learn more about the fascinating way our bodies work.

Although scientists know that the number of hours of daylight controls the actions of some living things, they know that this is not the only factor because

A．the fiddler crab’s shell reached the darkest color at the same time in Cape Cod, Massachusetts and its neighboring island
B．the fiddler crab continued to change color in the dark
C．plants will not grow without sunlight
D．all of the above