托福閱讀真題Official 54 Passage 3（二）

托福閱讀真題Official 54 Passage 3（二）

Elements of Life

The creation of life requires a set of chemical elements for making the components of cells.Life on Earth uses about 25 of the 92 naturally occurring chemical elements,although just 4 of these elements—oxygen,carbon,hydrogen,and nitrogen—make up about 96 percent of the mass of living organisms.Thus,a first requirement for life might be the presence of most or all of the elements used by life.

Interestingly,this requirement can probably be met by almost any world.Scientists have determined that all chemical elements in the universe besides hydrogen and helium(and a trace amount of lithium)were produced by stars.These are known as heavy elements because they are heavier than hydrogen and helium.Although all of these heavy elements are quite rare compared to hydrogen and helium,they are found just about everywhere.

Heavy elements are continually being manufactured by stars and released into space by stellardeaths,so their amount compared to hydrogen and helium gradually rises with time.Heavy elements make up about 2 percent of the chemical content(by mass)of our solar system;the other 98 percent is hydrogen and helium.In some very old star systems,which formed before many heavy elements were produced,the heavy-element share may be less than 0.1 percent.Nevertheless,every star system studied has at least some amount of all the elements used by life.Moreover,when planetesimals—small,solid objects formed in the early solar system that may accumulate to become planets—condense within a forming star system,they are inevitably made from heavy elements because the more common hydrogen and helium remain gaseous.Thus,planetesimals everywhere should contain the elements needed for life,which means that objects built from planetesimals—planets,moons,asteroids,and comets—also contain these elements.The nature of solar-system formation explains why Earth contains all the elements needed for life,and it is why we expect these elements to be present on other worlds throughout our solar system,galaxy,and universe.

Note that this argument does not change,even if we allow for life very different from life on Earth.Life on Earth is carbon based,and most biologists believe that life elsewhere is likely to be carbon based as well.However,we cannot absolutely rule out the possibility of life with another chemical basis,such as silicon or nitrogen.The set of elements(or their relative proportions)used by life based on some other element might be somewhat different from that used by carbon-based life on Earth.But the elements are still products of stars and would still be present in planetesimals everywhere.No matter what kinds of life we are looking for,we are likely to find the necessary elements on almost every planet,moon,asteroid,and comet in the universe.

A somewhat stricter requirement is the presence of these elements in molecules that can be used as ready-made building blocks for life,just as early Earth probably had an organic soup of amino acids and other complex molecules.Earth’s organic molecules likely came from some combination of three sources:chemical reactions in the atmosphere,chemical reactions near deep-sea vents in the oceans,and molecules carried to Earth by asteroids and comets.The first two sources can occur only on worlds with atmospheres or oceans,respectively.But the third source should have brought similar molecules to nearly all worlds in our solar system.

Studies of meteorites and comets suggest that organic molecules are widespread among both asteroids and comets.Because each body in the solar system was repeatedly struck by asteroids and comets during the period known as the heavy bombardment(about 4 billion years ago),each body should have received at least some organic molecules.However,these molecules tend to be destroyed by solar radiation on surfaces unprotected by atmospheres.Moreover,while these molecules might stay intact beneath the surface(as they evidently do on asteroids and comets),they probably cannot react with each other unless some kind of liquid or gas is available to move them about.Thus,if we limit our search to worlds on which organic molecules are likely to be involved in chemical reactions,we can probably rule out any world that lacks both an atmosphere and a surface or subsurface liquid medium,such as water.

Question 3 of 14

Why does the author provide the information that"Heavy elements...stellar deaths"?

A.To explain how it is that the elements required for life can be found everywhere

B.To provide evidence that our solar system is relatively young

C.To argue that some solar systems are more likely to support life than others

D.To explain why heavy elements have greater mass than hydrogen and helium

正確答案：A

題目詳解

題型分類：修辭目的題

題干分析：考察多個段落的關(guān)系。

選項分析：

提問句位于第三段首句，是二三段的過渡句。而第二段最后一句是they are found just about everywhere對應(yīng)A選項the elements required for life can be found everywhere。

B選項，relatively young無中生有，這一句沒有給出多少重元素含量算是年輕。

C選項，more likely to support life無中生有。

D選項，why heavy elements have greater mass無中生有。

Question 4 of 14

Paragraph 3 suggests that which of the following may be a difference between very old star systems and newer star systems?

A.Older star systems are likely to have fewer planets,moons,asteroids,and comets than newer star systems.

B.Newer star systems probably contain more hydrogen and helium than older star systems.

C.Newer star systems probably contain more heavy elements than older star systems.

D.The process of solar-system formation may have been fundamentally different in older star systems than in newer star systems.

正確答案：C

題目詳解

題型分類：推論題

原文定位：根據(jù)very old star systems and newer star systems定位到第三句。

選項分析：

C選項more heavy elements than older star systems對應(yīng)定位句In some very old star systems…the heavy-element share may be less than 0.1 percent。C選項正確。

A選項信息出自倒數(shù)第二句，但是沒有說fewer。

B選項，more hydrogen and helium than older star systems與定位句說老的行星重元素少，所以推測老的行星輕元素多，矛盾。

D選項，formation may have been fundamentally different無中生有。

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