秦始皇、汉武帝都是我国历史上有雄才大略的皇帝,他们采取的共同统治措施是( ) A . 对匈奴实行“和亲” B . 在长安设立太学 C . 抵御匈奴,巩固边防 D . 统一度量衡 答案:CThe fiddler crab (蟹) is a living clock. It
shows 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
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 regulated (受控制) 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!
1.The fiddler crab is like a clock because it
changes colour ______.
A in a regular 24—hour
rhythm
B. in answer to the sun’s rays
C. at low
tide
D.
every fifty minutes
2.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
3. 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
4.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
5.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