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： relative land-icevolume for a given period can be deduced from the ratioof two oxygen isotopes²， 16 and 18， found in ocean sedi-ments. Almost all the oxygen in water is oxygen 16， buta few molecules³ out of every thousand incorporate the（15） heavier isotope¹ 18. When an ice age begins， the conti-nental ice sheets grow， steadily⁴ 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 determined⁵ by analyzing⁶ ocean sediments⁸ of theperiod， because these sediments are composed of calciumcarbonate shells of marine¹⁰ organisms， shells that were（25） constructed with oxygen atoms drawn¹¹ from the sur-rounding ocean. The higher the ratio of oxygen 18 to oxygen 16 in a sedimentary specimen¹²， the more land icethere was when the sediment⁷ was laid down.
　　As an indicator¹³ of shifts in the Earth‘s climate， the（30） isotope record has two advantages. First， it is a globalrecord： there is remarkably¹⁴ little variation in isotoperatios in sedimentary specimens¹⁵ 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 fluctuations¹⁷ in global ice volume over the past several hundred thousand years（40） have a pattern： 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 volcanic¹⁸ particulates¹⁹ 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： changes in the Earth‘s orbitcan be calculated and dated by applying Newton’s laws（50） of gravity to progressively earlier configurations²⁰ 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 outdated²¹ 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） initiating²² a debate about a widely accepted theory

　　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 plausible²³ 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 promising²⁴ 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 lighter²⁵ 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 calcium⁹ 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 atmospheric²⁶ 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 isotopic²⁷ ratio as ocean water
　　　（B） less oxygen 18 than does ocean water
　　　（C） less oxygen 18 than has the ice contained in continental¹⁶ 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？
　　　Ⅰ. The last ice age occurred about 25，000 years ago.
　　　Ⅱ. Ice ages have lasted about 10，000 years for at least the last several hundred thousand years.
　　　Ⅲ. Ice ages have occurred about every 100，000 years for at least the last several hundred thousand years.
　　　（A） Ⅰ only
　　　（B） Ⅱ only
　　　（C） Ⅲ only
　　　（D） Ⅰand only
　　　（E） Ⅰ，Ⅱ and Ⅲ

　　7. It can be inferred from the passage that calcium　carbonate shells
　　　（A） are not as susceptible²⁸ to deterioration²⁹ 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