托福考试：TOEFL模拟试题
<p> <span word="Questions">Questions</span> 12-20</p><p> <span word="The">The</span> <span word="elements">elements</span> <span word="other">other</span> <span word="than">than</span> <span word="hydrogen">hydrogen</span> <span word="and">and</span> <span word="helium">helium</span> <span word="exist">exist</span> <span word="In">In</span> <span word="such">such</span> <span word="small">small</span> <span word="quantities">quantities</span> <span word="that">that</span> <span word="it">it</span> <span word="is">is</span> <span word="accurate">accurate</span> <span word="to">to</span> <span word="say">say</span> <span word="that">that</span> <span word="the">the</span> <span word="universe">universe</span> <span word="somewhat">somewhat</span> <span word="more">more</span> <span word="than">than</span> 25 <span word="percent">percent</span> <span word="helium">helium</span> <span word="by">by</span> <span word="weight">weight</span> <span word="and">and</span> <span word="somewhat">somewhat</span> <span word="less">less</span> <span word="than">than</span> 25 <span word="percent">percent</span> <span word="hydrogen">hydrogen</span>.</p><p> <span word="Astronomers">Astronomers</span> <span word="have">have</span> <span word="measured">measured</span> <span word="the">the</span> <span word="abundance">abundance</span> <span word="of">of</span> <span word="helium">helium</span> <span word="throughout">throughout</span> <span word="our">our</span> <span word="galaxy">galaxy</span> <span word="and">and</span> <span word="in">in</span> <span word="other">other</span> <span word="galaxies">galaxies</span> <span word="as">as</span> <span word="well">well</span>. <span word="Helium">Helium</span> <span word="has">has</span> <span word="been">been</span> <span word="found">found</span> <span word="In">In</span> <span word="old">old</span> <span word="stars">stars</span>, <span word="in">in</span> <span word="relatively">relatively</span> <span word="young">young</span> <span word="ones">ones</span>, <span word="in">in</span> <span word="interstellar">interstellar</span> <span word="gas">gas</span>, <span word="and">and</span> <span word="in">in</span> <span word="the">the</span> <span word="distant">distant</span> <span word="objects">objects</span> <span word="known">known</span> <span word="as">as</span> <span word="quasars">quasars</span>. <span word="Helium">Helium</span> <span word="nuclei">nuclei</span> <span word="have">have</span> <span word="also">also</span> <span word="been">been</span> <span word="found">found</span> <span word="to">to</span> <span word="be">be</span> <span word="constituents">constituents</span> <span word="of">of</span> <span word="cosmic">cosmic</span> <span word="rays">rays</span> <span word="that">that</span> <span word="fall">fall</span> <span word="on">on</span> <span word="the">the</span> <span word="earth">earth</span> (<span word="cosmic">cosmic</span> <span word="rays">rays</span> <span word="are">are</span> <span word="not">not</span> <span word="really">really</span> <span word="a">a</span> <span word="form">form</span> <span word="of">of</span> <span word="radiation">radiation</span>; <span word="they">they</span> <span word="consist">consist</span> <span word="of">of</span> <span word="rapidly">rapidly</span> <span word="moving">moving</span> <span word="particles">particles</span> <span word="of">of</span> <span word="numerous">numerous</span> <span word="different">different</span> <span word="kinds">kinds</span>). <span word="It">It</span> <span word="doesnt">doesnt</span> <span word="seem">seem</span> <span word="to">to</span> <span word="make">make</span> <span word="very">very</span> <span word="much">much</span> <span word="difference">difference</span> <span word="where">where</span> <span word="the">the</span> <span word="helium">helium</span> <span word="is">is</span> <span word="found">found</span>. <span word="Its">Its</span> <span word="relative">relative</span> <span word="abundance">abundance</span> <span word="never">never</span> <span word="seems">seems</span> <span word="to">to</span> <span word="vary">vary</span> <span word="much">much</span>. <span word="In">In</span> <span word="some">some</span> <span word="places">places</span>, <span word="there">there</span> <span word="may">may</span> <span word="be">be</span> <span word="slightly">slightly</span> <span word="more">more</span> <span word="of">of</span> <span word="it">it</span>; <span word="In">In</span> <span word="others">others</span>, <span word="slightly">slightly</span> <span word="less">less</span>, <span word="but">but</span> <span word="the">the</span> <span word="ratio">ratio</span> <span word="of">of</span> <span word="helium">helium</span> <span word="to">to</span> <span word="hydrogen">hydrogen</span> <span word="nuclei">nuclei</span> <span word="always">always</span> <span word="remains">remains</span> <span word="about">about</span> <span word="the">the</span> <span word="same">same</span>.</p><p> <span word="Helium">Helium</span> <span word="is">is</span> <span word="created">created</span> <span word="in">in</span> <span word="stars">stars</span>. <span word="In">In</span> <span word="fact">fact</span>, <span word="nuclear">nuclear</span> <span word="reactions">reactions</span> <span word="that">that</span> <span word="convert">convert</span> <span word="hydrogen">hydrogen</span> <span word="to">to</span> <span word="helium">helium</span> <span word="are">are</span> <span word="responsible">responsible</span> <span word="for">for</span> <span word="most">most</span> <span word="of">of</span> <span word="the">the</span> <span word="energy">energy</span> <span word="that">that</span> <span word="stars">stars</span> <span word="produce">produce</span>. <span word="However">However</span>, <span word="the">the</span> <span word="amount">amount</span> <span word="of">of</span> <span word="helium">helium</span> <span word="that">that</span> <span word="could">could</span> <span word="have">have</span> <span word="been">been</span> <span word="produced">produced</span> <span word="in">in</span> <span word="this">this</span> <span word="manner">manner</span> <span word="can">can</span> <span word="be">be</span> <span word="calculated">calculated</span>, <span word="and">and</span> <span word="it">it</span> <span word="turns">turns</span> <span word="out">out</span> <span word="to">to</span> <span word="be">be</span> <span word="no">no</span> <span word="more">more</span> <span word="than">than</span> <span word="a">a</span> <span word="few">few</span> <span word="percent">percent</span>. <span word="The">The</span> <span word="universe">universe</span> <span word="has">has</span> <span word="not">not</span> <span word="existed">existed</span> <span word="long">long</span> <span word="enough">enough</span> <span word="for">for</span> <span word="this">this</span> <span word="figure">figure</span> <span word="to">to</span> <span word="he">he</span> <span word="significantly">significantly</span> <span word="greater">greater</span>. <span word="Consequently">Consequently</span>, <span word="if">if</span> <span word="the">the</span> <span word="universe">universe</span> <span word="is">is</span> <span word="somewhat">somewhat</span> <span word="more">more</span> <span word="than">than</span> 25 <span word="percent">percent</span> <span word="helium">helium</span> <span word="now">now</span>, <span word="then">then</span> <span word="it">it</span> <span word="must">must</span> <span word="have">have</span> <span word="been">been</span> <span word="about">about</span> 25 <span word="percent">percent</span> <span word="helium">helium</span> <span word="at">at</span> <span word="a">a</span> <span word="time">time</span> <span word="near">near</span> <span word="the">the</span> <span word="beginning">beginning</span>..</p>
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