.
Do you know the fiddler crab(招潮蟹) is a living clock? It shows the time of day by the color of its skin, which is dark by day and pale by night. The crab’s changing skin color follows a regular twenty-four hours cycle that exactly matches the daily rhythm of the sun.
Does the crab actually keep time, or does its skin simply respond to(对……反应) the sun’s rays, changing color according to the amount of light that strikes it? To find it out, biologists kept crabs in a dark room for two months. Even without daylight the crab’s skin color continued to change exactly on time.
This probably developed gradually in response to the rhythm of the sun, to help protect the crab from sunlight and enemies. After millions of years it has become completely regular inside the living body of the crab.
The biologists noticed that once each day the color of the fiddler crab is especially dark, and that each day this occurs fifty minutes later than on the day before. From this they discovered that each crab follows not only the rhythm of the sun but also that of the tides(潮汐). The crab’s period of greatest darkening is exactly the time of low tide on the beach where it was caught!
64. Which of the following statements is TRUE?
A. The crab cannot actually keep time.
B. The crab’s skin changes color according to the amount of light that strikes it.
C. The crab’s skin still changes color even if there isn’t any daylight.
D. The crab likes the sun’s rays best of all.
65. The crab’s color-changing ability was probably developed _________.
A. by the work of biologists                     B. over plenty of dark nights
C. by protecting themselves from enemies          D. over millions of years
66. The biologists discovered that the crab’s darkest color occurs _________.
A. at the time of low tide                       B. when it’s dark
C. fifty minutes later than the sunset              D. every two days

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