B
The fiddler crab (蟹) is a living clock. It indicates the time of day by the colour of its skin, which is dark by day and pale by night. The crab’s changing colour follows a regular twenty-four hour plan that exactly matches the daily rhythm of the sun.
Does the crab actually keep time, or does its skin simply answer to the sun’s rays, changing colour according to the amount of light which strikes it? To find out, biologists kept crabs in a dark room for two months. Even without daylight, the crab’s skin colour continued to change exactly on time.
This characteristic probably developed gradually in answer to the daily rising and setting of the sun, to help protect the crab from sunlight and enemies. After millions of years it has become completely controlled inside the living body of the crab.
The biologists noticed that once each day the colour of the fiddler crab is especially dark, and that each day this happens 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 (潮水). The crab’s period of greatest darkening is exactly the time of low tide on the beach where it was caught!
40. The crab’s changing colour _______.
A. tells the crab what time it is 
B. protects the crab from the sunlight and enemies
C. keeps the crab warm            
D. is of no real use
41. When the fiddler crabs were kept in the dark, they _______.
A. did not change colour          B. changed colour more quickly
C. changed colour more slowly     D. changed colour on the same timetable
42. The crab’s colour—changing ability was probably developed _______.
A. in the process of evolution (进化)   B. over millions of years
C. by the work of biologists          D. both A and B  
43. The best title for this selection would be _______.
A. The Sun and the Tides             B. Discoveries in Biology
C. A scientific Study                 D. A Living Clock