With what topic is the passage mainly concerned、

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• 1Which is implied but not stated、

  A.Precise timekeeping is essential in science.

  B.Scientists expect to disprove Einstein's relativity theories.

  C.Atomic clocks will be important in space flight.

  D.The rate of vibration of an atom never varies.

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• 2Identical atomic clocks may be used to check_____

  A.the accuracy of predictions based on theories of relativity

  B.all of Einstein's theories

  C.the effect of outer space on an atomic clock

  D.the actual speed of an orbiting satellite

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• 3From the selection, we may assume that temperature changes _______

  A.affect only ammonia molecules

  B.may affect the vibration rate of atoms

  C.affect the speed at which atoms travel

  D.do not affect atoms in any way

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• 4Scientists expect that the atomic clocks will be_______.

  A.more durable

  B.indestructible

  C.more precise

  D.absolutely accurate

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• 5From the facts given in this article we can tell that _______

  A.often a form of energy is useless to us until it becomes another form

  B.steam power is cheaper and better than electrical power

  C.in the near future, there will be enough nuclear power to supply all our energy needs

  D.energy can change to another form, but the new form cannot change hack into the old form

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• 6It can be inferred from the passage that insects do not have _______.

  A.nerve cells

  B.reflexes

  C.brains

  D.Backbones

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• 7According to the passage, what helps to coordinate the complex physical activity of a mammal、

  A.The cerebrum

  B.The midbrain

  C.The cerebrum

  D.The forebrain

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• 8In the last paragraph, the phrase "took to" could best be replaced by which of the following、

  A.began

  B.fled to

  C.carried to

  D.became friendly toward

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