passage 18
¡¡¡¡milankovitch proposed in the early twentieth centurythat the ice ages were caused by variations in the earth¡®sorbit around the SUN. for sometime this theory wasconsidered untestable£¬ largely BECause there was no suffi-£¨5£© ciently precise chronology of the ice ages with which£  the orbital variations could be matched.
¡¡¡¡to establish such a chronology it is necessary todetermine the relative amounts of land ice that existedat various times in the earth¡®s past. a recent discovery£¨10£© makes such a determination possible£º relative land-icevolume for a given period can be deduced from the ratioof two oxygen isotopes£¬ 16 and 18£¬ found in ocean sedi-ments. almost all the oxygen in water is oxygen 16£¬ buta few molecules out of every thousand incorporate the£¨15£© heavier isotope 18. when an ice age begins£¬ the conti-nental ice sheets grow£¬ steadily reducing the amount ofwater evaporated from the ocean that will eventuallyreturn to it. because heavier isotopes tend to be leftbehid when water evaporates from the ocean surfaces£¬£¨20£© the remaining ocean water becomes proGREssivelyenriched in oxygen 18. the degree of enrichment canbe determined by analyzing ocean sediments of theperiod£¬ because these sediments are composed of calciumcarbonate shells of marine organisms£¬ shells that were£¨25£© constructed with oxygen atoms drawn from the sur-rounding ocean. the higher the ratio of oxygen 18 to oxygen 16 in a sedimentary specimen£¬ the more land icethere was when the sediment was laid down.
¡¡¡¡as an indicator of shifts in the earth¡®s climate£¬ the£¨30£© isotope record has two advantages. first£¬ it is a globalrecord£º there is remarkably little variation in isotoperatios in sedimentary specimens taken from differentcontinental locations. second£¬ it is a more continuousrecord than that taken from rocks on land. because of£¨35£© these advantages£¬ sedimentary evidence can be datedwith sufficient accuracy by radiometric methods toestablish a precise chronology of the ice ages. the datedisotope record shows that the fluctuations in global ice volume over the past several hundred thousand years£¨40£© have a pattern£º an ice age occurs roughly once every100£¬000 years. these data have established a strongconnection between variations in the earth¡¯s orbit andthe periodicity of the ice ages.
¡¡¡¡however£¬ it is important to note that other factors£¬£¨45£© such as volcanic particulates or variations in the amountof sunlight received by the earth£¬ could potentially haveaffected the climate. the advantage of the milankovitchtheory is that it is testable£º changes in the earth¡®s orbitcan be calculated and dated by applying newton¡¯s laws£¨50£© of gravity to progressively earlier configurations of the£  bodies in the solar system. yet the lack of informationabout other possible factors affecting global climate doesnot make them unimportant.

¡¡¡¡1. in the passage£¬ the author is primarily interested in
¡¡¡¡£  £¨a£© suggesting an alternative to an outdated research method
¡¡¡¡£  £¨b£© introducing a new research method that calls an accepted theory into question
¡¡¡¡£  £¨c£© emphasizing the instability of data gathered from the application of a new scientific method
¡¡¡¡£  £¨d£© presenting a theory and describing a new method to test that theory
¡¡¡¡£  £¨e£© initiating a debate about a widely accepted theory {C?N3€~L#Nzd$28 [ ±¾ ×Ê ÁÏ À´ Ô´ ÓÚ ¹ó ÖÝ Ñ§ Ï° Íø ÍâÓïÔ°µØGMAT http://Www.gzU521.com ] {C?N3€~L#Nzd$28

¡¡¡¡2. the author of the passage would be most likely toagree with which of the following statements aboutthe milankovitch theory£¿
¡¡¡¡£  £¨a£© it is the only possible explanation for the ice ages.
¡¡¡¡£  £¨b£© it is too limited to provide a plausible explanation for the ice ages£¬ despite recent research findings.
¡¡¡¡£ £¨c£© it cannot be tested and confirmed until further research on volcanic activity is done.
¡¡¡¡£  £¨d£© it is one plausible explanation£¬ though not the£ only one£¬ for the ice ages.
¡¡¡¡£ £¨e£© it is not a plausible explanation for the ice ages£¬ although it has opened up promising possibilities for future research.

¡¡¡¡3. it can be inferred from the passage that the isotope£ record taken from ocean sediments would be less useful£ to researchers if which of the following were true£¿
¡¡¡¡£ £¨a£© it indicated that lighter isotopes of oxygen predominated at certain times.
¡¡¡¡£ £¨b£© it had far more gaps in its sequence than the record taken from rocks on land.
¡¡¡¡£ £¨c£© it indicated that climate shifts did not occur every 100£¬000 years.
¡¡¡¡£ £¨d£© it indicated that the ratios of oxygen 16 and oxygen 18 in ocean water were not consistent with those found in fresh water.
¡¡¡¡£ £¨e£© it stretched back for only a million years.

¡¡¡¡4. according to the passage£¬ which of the following is true of the ratios of oxygen isotopes in ocean sediments£¿
¡¡¡¡£ £¨a£© they indicate that sediments found during an ice age contain more calcium carbonate than sediments formed at other times.
¡¡¡¡£ £¨b£© they are less reliable than the evidence from rocks on land in determining the volume of land ice.
¡¡¡¡£ £¨c£© they can be used to deduce the relative volume of land ice that was present when the sediment was laid down.
¡¡¡¡£ £¨d£© they are more unpredictable during an ice age than in other climatic conditions.
¡¡¡¡£ £¨e£© they can be used to determine atmospheric conditions at various times in the past.

¡¡¡¡5. it can be inferred from the passage that precipitation£ formed from evaporated ocean water has
¡¡¡¡£ £¨a£© the same isotopic ratio as ocean water
¡¡¡¡£ £¨b£© less oxygen 18 than does ocean water
¡¡¡¡£ £¨c£© less oxygen 18 than has the ice contained in continental ice sheets
¡¡¡¡£ £¨d£© a different isotopic composition than has precipitation formed from water on land
¡¡¡¡£ £¨e£© more oxygen 16 than has precipitation formed from fresh water

¡¡¡¡6. according to the passage£¬ which of the following is £¨are£© true of the ice ages£¿
¡¡¡¡£ ¢¡. the last ice age occurred about 25£¬000 years ago.
¡¡¡¡£ ¢¢. ice ages have lasted about 10£¬000 years for at least the last several hundred thousand years.
¡¡¡¡£ ¢£. ice ages have occurred about every 100£¬000 years for at least the last several hundred thousand years.
¡¡¡¡£ £¨a£© ¢¡ only
¡¡¡¡£ £¨b£© ¢¢ only
¡¡¡¡£ £¨c£© ¢£ only
¡¡¡¡£ £¨d£© ¢¡and only
¡¡¡¡£ £¨e£© ¢¡£¬¢¢ and ¢£

¡¡¡¡7. it can be inferred from the passage that calcium£ carbonate shells
¡¡¡¡£ £¨a£© are not as susceptible to deterioration as rocks
¡¡¡¡£ £¨b£© are less common in sediments formed during an ice age
¡¡¡¡£ £¨c£© are found only in areas that were once covered by land ice
¡¡¡¡£ £¨d£© contain radioactive material that can be used to determine a sediment¡®s isotopic composition
¡¡¡¡£ £¨e£© reflect the isotopic composition of the water at the time the shells were formed

¡¡¡¡8. the purpose of the last paragraph of the passage is to
¡¡¡¡£ £¨a£© offer a note of caution
¡¡¡¡£ £¨b£© introduce new evidence
¡¡¡¡£ £¨c£© present two recent discoveries
¡¡¡¡£ £¨d£© summarize material in the preceding paragraphs
¡¡¡¡£ £¨e£© offer two explanations for a phenomenon

¡¡¡¡9. according to the passage£¬ one advantage of studying the isotope record of ocean sediments is that it
¡¡¡¡£ £¨a£© corresponds with the record of ice volume taken from rocks on land
¡¡¡¡£ £¨b£© shows little variation in isotope ratios when samples are taken from different continental locations
¡¡¡¡£ £¨c£© corresponds with predictions already made by climatologists and experts in other fields
¡¡¡¡£ £¨d£© confirms the record of ice volume initially established by analyzing variations in volcanic emissions
¡¡¡¡£ £¨e£© provides data that can be used to substantiaterecords concerning variations in the amountof sunlight received by the earth