掌握了gre阅读里的长难句，到了实战演习的时候了。gre阅读练习每日一篇帮助gre考生循序渐进地进行练习和总结。希望gre考生在进行gre阅读练习时，也按着考试时候的时间规定自己的练习，这样才能有效果。
Although scientists observe that an organism’s behavior falls into rhythmic patterns, they disagree about how these patterns are affected when the organism is transported to a new environment. One experimenter, Brown, brought oysters from Connecticut waters to Illinois waters. She noted that the oysters initially opened their shells widest when it was high tide in Connecticut, but that after fourteen days their rhythms had adapted to the tide schedule in Illinois. Although she could not posit an unequivocal causal relationship between behavior and environmental change, Brown concluded that a change in tide schedule is one of several possible exogenous influences (those outside the organism) on the oysters’ rhythms. Another experimenter, Hamner, however, discovered that hamsters from California maintain their original rhythms even at the South Pole. He concluded that endogenous influences (those inside the organism) seem to affect an organism’s rhythmic behavior.
17. All of the following could be considered examples of exogenous influences on an organism EXCEPT the influence of the
(A) level of a hormone on a field mouse’s readiness for mating
(B) temperature of a region on a bear’s hibernation
(C) salt level of a river on a fish’s migration
(D) humidity of an area on a cat’s shedding of its fur
(E) proximity of an owl on a lizard’s searching for food
18. Which of the following statements best describes the conclusion drawn by Brown (lines 14-17)
(A) A change in tide schedule is the primary influence on an oyster’s rhythms.
(B) A change in tide schedule may be an important exogenous influence on an oyster’s rhythms.
(C) Exogenous influences, such as a change in tide schedule, seldom affect an oyster’s rhythms.
(D) Endogenous influences have no effect on an oyster’s rhythms.
(E) Endogenous influences are the only influences on an oyster’s rhythms.